4 traped doors, one pressure plate, and a tnt block.
Place the doors in a square fashion, and the pressure plate in the middle so when you go in it it locks you in.
Everyone knows that trick and knows how to get out, of course. But, the thing is place a tnt block 1 block below the block the pressure plate is, guys comes in "Hehe look at this stupid noob trap. I'ma go in it and get out. -goes in- ...do I hear tn-" Nuff said.
/me explodes.