What about sword blades that are sticking into the wall, you don't see them. then when you walk throught the corridor and triggger the trigger *swoosh* the blade swings out in an arc and decapitates anyone that wasn't ducking. Or cuts out anyone's feet from who didn't jump in time.
I like the suck-into-fan thing mentioned above , that's great.
Also trap doors are always fun. That open up when stand on and drop you in a variety of hazards, like lave, spike, etc.
Maybe a room like that map in UT where the air got sucked out and anyone trpped inside was going to explode in a load of giblets. That could be real fun.
Or a rocket engine test room ala HL. Where you could toast your nemesis.
BTW, im working on a dualing map right now. I'll inform some more when it's ready for some testing. Hope this week.
cheers.