Landmines and TNT traps and such things of explosive might

[TehCthulhu]

Too bad there's no way to get this to work, not that I can think of at least.

[P]
[L][L*][L]
[L][T][L]
[L][W][L]
[L][W][L]
[L][L][L]
P = pressure plate
T = tnt
L = any solid block
L* = any solid block that falls when unsupported (e.g. sand)
W = water

Or maybe if instead of the falling block, there was a block held out by a stick piston and triggering the pressure plate would break the circuit holding it out, causing it to retract under a wall or something opening the pit, or doping the person in with the TNT.