One good practical joke that I've seen done to somebody:
1) Take an old record album cover.
2) Fill the insides with shaving cream.
3) Place it halfway under a locked door to the victim's office, home, room,
etc. with the open end inside the room.
4) Jump on it.
Another, if you can get access to the victim's key chain is to switch all
his keys for keys that look exactly the same, but don't fit the locks he's
trying to open. If you can be around for this one, it's much more fun to
watch the person go crazy as he cannot open anything he owns.