Since I don't work outside the house, I figure I will need to cause some mischief around the house. A longtime favorite of mine, since it causes so much yelling and swearing, is the rubberband-on-the-dish-scrubber-trigger. When people get up to go to work at 5am, and go to use the sink, it never fails to soak them to the bone. And the great part is, it is usually after they have already dressed for work, so it's a double whammy. I know it isn't original (though I was pretty proud when I thought of it on my own back in '85), but it gets funnier each year, since nobody seems to learn their lesson.
Now I am contemplating smaller things to do to the kids (ages 4 and 7), since I don't want to scar them for life. Well, not much, at least...