Sorry, had to.
Now for the serious suggestions.
Fishing line tripwires attached to those little cheap hand-held alarms serve as really good (and annoying) "mines".
You could also use the same type of alarm system you find in many stores. The ones that beep as you cross the door or cut off an infrared laser beam?
A small box (film canister) on top of a door filled with cheap-ass .12 ammo that would be rigged to fall onto person entering an area.
That's all that comes to mind which is safe, cheap, and works well.