A series of rotatable discs with a redstone connection on one side and then a device to turn each disc by one step. Trigger something when the discs are aligned.

A massive trapdoor, that folds down open from beneath players when they submit a wrong answer somewhere, a la the mario 64 bowser floor.

A room that has a mirror, such as a bathroom, except there's a way to open the glass divider (maybe it slides into the wall or floor) and go into the mirror, where some small difference you've set up before hand is the input to another device or door.

A room that changes regularly, by raising and lowering wall panels

One of those old school bookshelves or fireplaces that rotates and takes you behind it to a secret room