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.
Sometimes being random is a good thing. Most of the time being funny is a great thing, so these random funny pictures and a good, great thing. Enjoy both the randomness and and humor of these pictures.