This prank is one to build up to April Fools Day. Every time you are near someone leave pennies / small coins without them noticing.
Leave a ton of pennies everywhere. If you know where they work, wait till they go out for lunch. Throw a few pennies on the floor of their office. Slip a few into their coat. If they leave the window on their car cracked, shove a few pennies inside it.
If you are ever in their house, leave pennies in really weird places so that they will be discovered eventually. Pennies in the fridge. Pennies inside of their socks. If you can do it sneakily enough, put pennies into their pockets.
But never let them find out it's you.
More April Fool Pranks
This one is great to prank your colleagues who work on a computer during April Fools' Day. Switch their computer screen upside down during a break.
Plants in your coworker's keyboard
This prank needs some preperation but is really worth it. Your coworker will show up and his keyboard will be full of little grasplants!
Trick your colleagues with a piece of tape
This is a great joke to trick your colleagues and this one is deviously simple. You only need a piece of tape.