A wonderful prank to pull at an office or a construction site is to lock the bathroom door.
Do not do it when anyone is in there!
Run a bolt through the lock (outhouses always have a lock hasp) or some plastic ties. If you are truly evil, use a padlock. This is especially fun if you have been protesting a construction project and you do not have the desire to do something destructive.
If you truly want to be evil, run a bead of epoxy that works with HDPE (High Density Poly Ethylene) around the edge of the door and seal it shut.