dress up in all black and run outside with a laptop or some expensive electronic when they come
pour warm milk over the bathroom floor right before they come home, wouldn't want it to go bad or cool down-you know what they'll think it is
somehow manage to make the doorknob electrocute a bit
pour soap all over the floor (this can hurt them so make sure to engineer some safety thing)
put lubricant in hand sanitizer (haha, this is evil)
get a stuff TON of marbles and pour them out when they enter the house
put a sticky note on the bottom of a computer mouse, and watch as they call tech support to get it fixed