Think rat traps. The springs may be loaded and rigged up so that when the lid of the present is lifted they all go off shooting confetti everywhere or whatever you want. I don't have time right now to draw up plans, otherwise i would. Maybe later.
i got an idea you know the party favors, the thing with the string and confetti, have the string on the out side and make it appear that its a way to open the gift easer (like a pack of gum) and have the nozzle of the party favor facing towards them
only down side is you would need to find some type of wrapping paper that would be soft enough so that the confetti can come though.
Who gives refrigerators to their girlfriend? Psh, what a fag.
edit: Put a rattlesnake in the box and tell her to shake and guess what it is. She opens it, and surprise! Five feet of angry Diamondback hanging off her face. She'll probably die or at least be disfigured for life, but she sure as hell will be surprised when she opens that box.
Fill it with spiders.
Fill it with deadly snakes.
Fill it with mustard gas
Put a can of mace in it rigged to go off.
Put a stun gun in it.
The easiest way would be, of course, to hit him with the stun gun and then piss on him as he convulses, but that wouldn't be very clever. But don't argue with results.