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.
The perfect tattoo design for you and your lover if you have animal representations of yourselves, just change one of the animals into the creature that best represents you and do the same with the other animal except for your significant other.