Please note the following may not actually that safe. But what good prank is?
Freeze cans of shaving foam. Carefully cut open the cans and remove the frozen foam cylinder. Keep in a freezer bag and, during your day, deposit them in a rooms as you leave them. They will gradually melt, expand, and leave loads of foam behind.
Works best in small spaces such as cupboards, lockers, cars, and desk drawers.