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.
This is the 'mirror fence' designed and installed by artist Alyson Shotz. It's a picket fence made with mirrored surfaces to blend into the environment and become less obtrusive. I prefer obtrusive fences. My house is surrounded by.