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.
How you are seen by: Yourself - Your mom - Your dad - Your big brother - Your tiny brother - Your crush - The girl you don't like - Your grandma - Your dog - Your president - Your teacher - A blind person