Why wait for April 1st? At work we play pranks all the time.
Wait 'till someone walks away and rub grease on all of their tools or the handle of their box.
When someone is in "the office", fill the cheetah (air tank with a large spout and quick release valve used for seating the beads on tires), place the end of the nozzle under the bathroom door and hit the button.
*Disclaimer and Warning* If you have tiles on the floor, do this when the room is empty, so they don't get injured with flying tiles...
When I was a welder, we would tack each others tools to the work table.
Put their car on blocks, remove the wheels and hide them among 1400 tires in the stock room.
If they drive a pick-up, remove the tailgate and hide it in the bushes.
I have many more that I will have to post later, I have to go pick Ms. Ratman up.