Odd and ends! Bits and bobs! Stuff! You can find these things here. (And if you have more to point out to me, feel free in the comments. I have no doubt I missed things of interest.)
- Try naming your character as people from the game. In many cases you'll be outright refused, mainly in regards to major characters - Sans, Toriel, Flowey, and Gaster in particular - and in others you'll be allowed, though not without comment. Entering the name 'Frisk' will also trigger Hard Mode for the duration of the Ruins, bringing in enemies from the Core.
- During the, ah, 'fight' against the Puppet, you can get a range of different responses from Toriel, depending on how you deal with the situation. Whiffing your hits until the battle ends gives the funniest responses. (Thanks to Florelyang in the comments for pointing this one out. And no, killing the Puppet does not count against a Pacifist run, so don't worry.)
- You can 'spot' Toriel behind the pillar in one of the first rooms of the Ruins when she runs off to hide, before she hops out to surprise you. Ellipses!
- The first proper fight you'll hit is a battle against a Froggit. If you do anything but kill it with attacks Toriel will show up and glare it into leaving.
- If you wait in the room where Toriel told you to wait, she will, eventually, give you a phone call.
- In one of the rooms you'll find three Froggits. The first seemingly won't be there if you've spent a lot of time skipping through text, which makes sense since he hates when people do that. Another mentions that there are four Froggits in the room; the fourth is hidden in the cracks on the walls. The last allows you to change the font colour of Spare-able enemies from yellow back to white, or to pink.
- Whether deliberately or not, Napstablook's theme appears to sample some of the midi sound effects from EarthBound, most specifically Venus' singing voice.
- There's a spider bake sale in the Ruins. Purchase one of their food items and keep it and you can skip a battle much later on in the game.
- A minor one, but if you opt to sleep in your bed after you reach Toriel's Home she'll leave Butterscotch Pie on the floor for you. This is an extremely good healing item, and should be saved for the final battle against Asgore. You'll also get a brief message from sleeping in the bed. (Not sure if this only happens before or after the fight with Toriel. I'm pretty certain I got it before, but Kristen in the comments says after. Try both? It's not like it's a long walk.)
- It's virtually impossible to lose the fight against Toriel, as she'll purposely miss when your health gets too low. You can die, but you need to lose a lot of health during her last attack before she switches to non-lethal fireballs. Her expression when you do manage to die is priceless. (If heartbreaking.)
- Also during the Toriel battle, you can end it in a single hit if you attack her after Sparing her into giving up - but slicing her just before the battle's going to end. Ouch. No Mercy run after all?
- If you backtrack through rooms in the Ruins - in particular, the room with the Froggits - you may see Flowey disappearing into the floor on the edge of the screen. This is true of other areas in the game, as well.
- Eventually you'll receive some new equipment, allowing you to unequip the Dirty Bandage that starts as your default armour. You can thereafter use the Dirty Bandage as a normal item to restore 10 HP. (Thanks to Charlie Fire for pointing this one out in the comments.)
- You can use the Stick (the first weapon) to appease the many doggish monsters in this region. Funny times. It needs to be used as a normal item for this to work, not as a weapon.
- There are a number of cameras in Snowdin in hidden places. The most prominent is in a boulder right beside the exit from the Ruins.
- If you reload after first speaking with Sans and then speak with him again you'll get some alternate dialogue.
- There are usually alternate ways out of Papyrus' puzzles. On the coloured floor tile trap, for example, you can keep badgering Papyrus about the rules until the machine breaks down. On a Genocide run you will pretty much automatically skip all of the puzzles, because your character just doesn't give a toss about Papyrus' silliness.
- Speaking of the coloured floor trap puzzle, take a gooooooood, loooooong look at the machine that activates the trap. Does it look familiar? It should, if you've already played through the game once.
- Lesser Dog, a one-of-a-kind enemy that usually appears near a big patch of slidey ice, grows an insanely long neck if you keep Petting him. Spare him once you're done and you'll find him later, building snow sculptures - and the size of their necks will reflect how much you amazed the poor pup.
- Partway through Snowdin you'll find a snowman who will give you a Snowman Piece. If you keep this with you on a Neutral run and beat the game with it in your inventory you'll get an extra message at the end. During a Pacifist run you can get some different dialogue from him with it in your inventory, as well. During a Genocide run you just steal most of his body, resulting in a terrible collapse - and three Snowman Pieces. You. Monster.
- For that matter, if you Spare any / all of the dog enemies in Snowdin, you can find them again later at Grillby's.
- Not far from the long bridge leading to Snowdin Village is a side area with a mysterious cave, and inside that an even more mysterious door. This door leads to the Developer's Room. The only way to open it is to avoid all of the names in the 'Special Thanks' mini game that pops up at the end of a Pacifist run. As far as I know, it remains open on all subsequent runs as well.
- There's a wolf in Snowdin who hurls ice into the nearby lake. You'll see this ice travelling throughout the game, all the way to a dumping point in the Core. The only way to talk to this wolf is to complete a Pacifist run, then head to where he was working during the round-the-world tour that is the ending.
- If you head north through Snowdin Village you can find the launching spot for the boat man's ferry later in the game. It's vacant when you first arrive, but if you haven't fought Papyrus yet you can get a 'wrong number' phone call. It's... yep.
- You can't get a Game Over fighting Papyrus. If you lose he'll shove you in his shed. Keep losing and eventually he'll give you the option to stop fighting. (Seriously, though, he's not that hard.)
- If you Flirt with Papyrus the option to 'hang out' with him after the battle changes into a date. Both are pretty funny.
- During the hang out / date with Papyrus you can visit his kitchen, and when you do so he'll rush in to chat with you about it. Keep entering and exiting the kitchen for some humorous results, 'cause Papyrus has trouble keeping up with you.
- If you kill Papyrus you won't see Sans again until the very end of the game. He won't be happy with you.
- If you go behind Sans' and Papyrus' house you can find a locked door. The key to it is in Sans' room, and the key to Sans' room is... on Sans. You can get it much later in the game. More on that below.
- Make a bridge of Bridge Seeds to the sign in the second area where you must use them and you'll be told that you're a spectacular failure. To the right of this spot is another potential place to lay down the Seeds; this will take you to a bench where you'll find an Abandoned Quiche.
- If you take an umbrella while speaking to the Nice Cream guy, he'll knock 10 gold off of his normal asking price. 'brella buddies!
- There's a little purple dude who will spirit you back across the big gap in the water you crossed on a raft. He doesn't appear until after you leave the area, however.
- If you play Napstablook's music and leave his house it will remain in the background. Get into a fight and you'll get a one-off, hilarious enemy encounter.
- Napstablook will invite you to lay down if you speak to him in his house. Do so and wait long enough for a trippy, ethereal experience.
- Try refusing to give a Temmie a Temmie Flake. Just try it.
- Speaking of Temmies, and Temmie Flakes, you can find the Temmie Village on the mushroom-hidden paths two screens east of the turtle vendor. It's a speshul place. (And the only place where you can sell items.)
- Not sure if this counts as an 'easter egg', but you can sell the Clouded Glasses from the turtle vendor to the Temmie vendor in Temmie Village at a 20 gold profit. It takes a while, but this back-and-forth method grants you practically unlimited gold, assuming you have patience. (The upgraded Cell Phone with two Interdimensional Boxes helps, too.)
- The Temmie vendor will occasionally get really excited about an item you try to sell him, and will offer more than the normal asking price. Refuse him twice and his dialogue will... change. It gets even more insulting after you've sent him off to college.
- The battle against Undyne is a stylistic nod to the battle against Magus in Chrono Trigger in many respects - the battlefield, the setup, the atmosphere, even the dialogue on the save point. There's also a song on the Undertale OST called 'But the Earth Refused to Die', which is similar to Chrono Trigger's 'But the Future Refused to Change'. (Thanks to Sandact in the comments for pointing it out.)
- If you Challenge Undyne she'll increase the speed of her attacks. Plead with her and she'll occasionally reduce the speed of her attacks.
- If you kill Undyne, the 'Welcome to Hotland' sign at the end of Waterfall will not work.
- While 'hanging out' with Undyne you can point her spear at more than just the objects arrayed on her counter. Point at her for some funny dialogue. (Why does everyone seem to view this tiny child as a romantic? Must be a monster thing.)
- You can drain the water from the water cooler at the beginning of Hotland and dump it onto the ground. A tree grows here during the Pacifist ending if you do this.
- You'll encounter a mysterious boat man south of the first save point in Hot Land who can ferry you between the game's three major sections. He occasionally gives out hints about important Undertale secrets during the trips.
- Alphys will quietly give you the answers to Mettaton's quiz during your first battle. Watch her hands.
- Speaking of Mettaton, it is impossible to die during any of his 'shows'. Alphys will somehow save the day every time. This is most striking when disabling the bombs, because the timer slows down substantially when you reach the final ten seconds.
- Assuming you didn't kill Papyrus, you can find Sans selling Hot Dogs...? in Hotland. Buy so many that you have no more room in your inventory and Sans will start stacking them on your head, up to a maximum of thirty. Yes, thirty dirty dogs.
- There's a secret boss called So Sorry near the spot where you can purchase the Hot Dogs. Check the northern boundaries of the next screen to the west, just north of the intersection that goes south. You need to come to the secret room you'll find on the date and time listed on the sign for So Sorry to appear, however.
- You can skip Muffet's boss fight by purchasing one of her overpriced baked goods. You can also effectively skip her by using either Spider Cider or a Spider Donut at the beginning of the fight. Both of these items can be bought cheaply from the spider bake sale in the Ruins.
- At one point Mettaton begins to serenade you during an operatic solo number. This is a direct parody of a similar, slightly more poignant scene from Final Fantasy VI. (Thanks to Dibullba for pointing it out. Man, I can't believe I forgot that. I love FFVI.)
- If you killed the Royal Guards the Nice Cream man will continue to offer his wares just south of MTT Resort. Spare them, though, and they will buy up his entire stock. Oh well, love is more important.
- You can buy a Mystery Key from Bratty and Catty, in the alley beside the MTT Resort, for 600 gold. You can use this on the locked house next to Napstablook's in Waterfall to learn more about a certain robot.
- Using any of the food items from the MTT burger shack during the battle against Mettaton EX will give you a hearty boost to your ratings. The more expensive the item, the bigger the boost.
- Partway through the battle against Mettaton EX you'll be asked to write an essay about what you like best about Mettaton. A jumble of letters will earn you 100 points; writing 'legs' at any point will get you 350 points. Swear, however, and you'll lose points instead.
Home / True Lab / Ending
- If you close the program and reload it several times after receiving 'judgment' from Sans on a Pacifist run, he'll start giving you a series of code words. Keep doing this (preferably having saved at the save point just before Sans) and he'll eventually give you the key to his room, back in Snowdin Village. Don't save between visits with Sans - just turn the game off after you're done talking with him. (You can also do this on Neutral runs for different dialogue, but you won't get the key.)
- If you use the Butterscotch Pie during the battle against Asgore he'll receive a permanent penalty to his attack and defence. You can get similar results by talking to him repeatedly.
- The ending changes depending on which bosses you killed, as well as how many monsters you killed along the way.
- You can drop Undyne's Letter after she gives it to you (though she'll yell at you if you try to ditch it on the screen where she's standing). Go back and ask for another and you'll receive Undyne's Letter EX, which you cannot drop. It's too strong.
- You can sleep in one of the beds of the True Lab. Remain motionless for a while and a ghostly presence will reveal itself.
- During the epilogue of the Pacifist ending you can wander across the entirety of the world, and everyone (save perhaps one dude in Waterfall) will have something new to say, particularly the more important NPCs. You can even reenter the Ruins... and if you bother to walk aaaaaaall the way back to the beginning, you'll find the most informative NPC of them all waiting to gab with you.
Lurking in the background of all of this funky stuff is a spooky, mysterious dude named W.D. Gaster. He's a presence in Undertale whom you might completely miss if you're not sifting through the game with a fine-toothed comb. Gaster remains a major question mark for players of Undertale, and there's currently a manhunt-in-progress for the guy. I suggest checking out this highly-informative Steam page on Gaster, 'cause frankly, I got nuthin'.