Blue Teeth and a Numb Mouth A drop of food coloring or drop of Orajel on a toothbrush with alternately turn ones teeth blue or numb the mouth. Both temporary, both perfect April Fool’s day practical jokes.
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.