Planting a garden in a keyboard. Best office prank idea, especially if the co-worker goes on vacation for a week or so. I would use a keyboard that looks like theirs though. That way you don't get into trouble for destroying someone's property.
I don't understand why the American wants to call it "soccer" and their American football "football" when the players hold the ball in "hands" most of the time. This black and white ball is called "FOOTBALL"!