Anyways, I offer, for your enjoyment, some of my favorite trap ideas. I’ve used each of these in a game, with great results:
Trap Idea #1: Fake Door
For this trap, the characters need to make a successful search check. They will find a door, hidden behind a painting, curtain, or whatever is appropriate. The door is bright orange in color, and has hundreds of lights and levers of all colors. It cannot be forced open, and scrying or teleporting won’t work behind it. When anyone pulls a lever, everyone in the room takes 1 hit point of electrical damage. The lights then change color. The door will open when the characters push a red lever to the “down” position and a green lever to the “up” position.
To make this one especially interesting, use it while the characters are running from an encounter that they couldn’t overcome.
Trap Idea #2: Invisible Bridge
This is the old Indiana Jones trick. The players encounter a chasm that they have to cross. The characters have to cross the invisible bridge. The bridge is not invisible, but there are magics that make it imperceptible to some people. To do so, they need to first discover the bridge; provide a will save for each party member, difficult enough that some will miss it and not believe the bridge is there. It will be up to the remaining party members to convince the others that it is there. Once the party decides to cross, they will, of course, have to devise a way to get across. Make the bridge slippery, very narrow, place high winds, send in flying opponents, or whatever else you need to do to make it difficult. If your party has access to fly or teleport, make it a dead magic zone.
This really is probably best to do if you just hate your players. The scenario is the same as #2, but there’s really no bridge. Encourage characters that they need to all step onto the bridge together. Hilarity ensues.
Trap Idea #4: Exploding Coin
The characters enter into a room and see a single coin (or gem, or key, or whatever might entice them) laying on the floor. It can either be obvious, or you might have them make a search roll. When one of them picks it up, a delayed blast fireball of an appropriate strength for the party goes off.
Trap Idea #5: Stackable Pit Traps
This is your standard pit trap with a twist: if characters search the pit trap, they find another trap door in the floor, with hidden treasures inside. Alternatively, they find another trap door leading into another pit, and take even more damage. You pick.
Trap Idea #6: Wounded Enemy
The characters encounter an enemy who seems to be wounded. He is, of course, faking it, and will either lead the party to an ambush, or turn on them while they attempt to care for him. A variation is for the party to find a wounded ally set on betrayal or good-seeming creature, such as a fiendish unicorn.
Trap Idea #7: Alice in Wonderland
This is an oldie but a goodie. The characters enter a room with a table. On the table sits a potion that is labeled “drink me.” In the room is a mouse hole. Here’s where you make it interesting: in one scenario, the mouse hole is the only way out of a dungeon, and the characters have to shrink to get out. The problem will be undoing the effects of the shrinking potion. Maybe the mouse hole is a red herring; there are plenty of ways out of the dungeon, and the mouse hole doesn’t lead them out. In any case, once the characters are shrunken, they should have to fight the obligatory Giant Rat.
Trap Idea #8: Anti-gravity Pit
The characters come to the end of a hallway, only to find a vertical tunnel. The pit goes down as far as they can see, and 10′ up. When they enter the pit (probably with a rope) they are sucked up to the ceiling, which has a trap door with spikes or whatever other evils you can think of on the other side.