15 Pieces of Dungeon Flair

Here are 15 ideas for flair you can add to a dungeon:

1. Flumph plus Blind Monster with Good Sense of Smell -- So here's a good use of a flumph, despite what the dungeon bastard may think.  Place some random flumphs around a section of a dungeon, and then deeper into dungeon, place a blind, sleeping monster.  Let the party try to sneak past, and then BAM!  Spray from a Flumph.  The flumph gets eaten first and then the flumph's victim. Nice.
2. Two Invisibly Connected Pools and a Dire Crocodile -- There's nothing like a good game of cat and mouse between a PC and a dire crocodile.  Put two seemingly ordinary pools in an area (I used fountains) and then an invisible passage deep under the water.  Hide a crocodile in there.  Let him come up for a couple of bites, swim deep, and while the PCs are all looking away, surprise.  There is a crocodile biting your buttocks, sir!
3. The Trap within a Trap  -- Put in a difficult detect mundane trap, like a pit trap.  Then put an obvious lever.  "Hmmm... this must be how we disable it."  In actuality, pull the lever and yet another trap is sprung.  I prefer dropping weighted spears from hidden holes high above in the ceiling.  They make a nice sound when they hit.
4. Liars -- I used the standard "one always tells the truth, and one always lies" riddle the other day, and my PCs happened upon the other version in their paranoia: "both of us are liars and will make up interesting lies to see how much trouble we can get you into".  Of course, one of them can claim that one only lies and one only tells the truth, but that, too, can be a lie.  This can go a lot of ways, but a sphere of annihilation could be helpful.
5. A Dark Cloak -- A dark cloak hangs on the wall.  Oh wait, that is a cloaker.  Yikes!
6. Lights, Reflections, and All Shades of Gray  -- Take a funhouse hall of mirrors and add a bunch of highly localized light sources   You get all sorts of crazy reflections and shadows.  Now add a some real Shadows (the monster).  Oh, and make sure if a mirror gets broken that something really bad happens -- how about 6d6 electrical damage?
7. Help, They've Turned Me into a Farm Animal -- This one is great.  Put a caged animal somewhere in the dungeon, more convincingly near some sort of evil mage's realm.  "Help, they'v turned me into a cow? Just get this magic spell from *insert place* and you can turn me back!"  The catch is that the mage didn't turn them into a farm animal, they were a farm animal and the mage made them talk.  Now you have a human barbarian with the party with all the sense of a chicken, cow, pig, or other animal.  It's really great to do just before a careful attack.  And there goes the barbarian running into the room "Leroy Jenkiiiiiiinnnnnssss!"
8. Wands of Stupid -- This a general category.  Wands of stupid perform hilarious magic actions with little or no applicability the game world tactical scenario.  Examples include the Wand of Nothing, the Wand of Mustache (+1 to Disguise Checks), and the Wand of Skittles (shoots little fruit candies, enough to fill a room if you let it).  The players can have fun with these and they always give a good laugh.  Rarely, they an even be slightly helpful.
9. Acid Pits -- Fans of "Acquisitions, Incorporated" will appreciate the elegance of an acid pit.  The do a lot of damage (10d6 per round if you fall in) and can cause seemingly mundane skill checks, like climbing across a narrow ledge, to turn into matters of life and death.  And it is always fun to answer the question "Where did they get that much acid from?" with a monster in a later encounter.  "Ohhh... that's where they got the acid from!"
10. The Campsite from Hell -- Your party has found a wonderful room, with a secure door that can be readily braced.  There is a large hearth and firewood.  There might even be someplace nice to sleep, but once you and your crew goes to sleep, it's all going to change.  Incorporeal beings can come out of the woodwork, and for fun, let's put something big, mean, and nasty right outside the door, like a rhinoceros or an umber hulk.
11. Teleport Doors -- I hate when the convenience of a predrawn map gives the players too much info about the dungeon.  Instead of trying to draw on the fly, which can be tedious and time-consuming, predraw the map, but make a GM's key that tells where each door teleports to.  Now the players can't use the spatial layout of the map to figure out anything tactical.  Way to go, overused magic doors!
12. A Slide or Chute -- It doesn't take long for players to become very distrustful of scenarios.  Putting in a slide or a chute really plays off of this, by forcing the players to take a leap of faith.  For giggles, make one of the people that jump in disappear, and reappear elsewhere, perhaps imprisoned or captured by a creature for a late night snack.
13. Skill Check on the Run -- Normally skill checks aren't very exciting.  Put in a skill check with an encounter.  So rogue, how many rounds does it take you to pick this lock?  Oh by the way, we're being attacked by a bodak, so hurry up, eh?  Hey fighter, can you break down this door before the ceiling crushes us?
14. A Goat -- Players definitely catch on to subtlety.  Add a goat to the dungeon, a traditional animal of sacrifice.  Watch the suspense and paranoia build!  You an even add detailed descriptions of what the goat does to make it seem more significant.  Also, keep asking as a GM, "so where exactly is the goat?"
15. A Pop Culture Reference -- If you've ever played the Fallout franchise, you know how cool it is when you find pop culture references, especially geeky one, in the game.  A couple of weeks ago, I had some very tanned almost orange gnomes singing why they were carrying away a passed-out PC.  One of the players shouts out Oompah-Loompas.  The references are even better when they are a play-on-words.  The druid finds a bug and realizes that it a firefly and he says he is seeking serenity.