This is a very simple trick, but you'd be surprised just how effective it is. When the victim is sleeping, make a shallow paper tray and scotch tape it to his door AND to the door frame near the knob. Fill it with unpopped popcorn. When he opens the door (from the inside, obviously) he'll have popcorn flung all over the place. Not only does it pack quite a little surprise, it also takes forever to find all of the popcorn afterwards.
