We had a guy in our workshop and every second day, we'd shave a couple of mm off one of his chair legs. He was always complaining about his wobbly chair and the fact that the floor was so uneven, but he could never seem to get the levels right. It was only after a year that he cottoned on when someone who hadn't seen him for a while asked him if he'd shrunk or just looked smaller sitting down.
Another one was supergluing the coffee mugs to the workbench.
Ziptie a condom around the exhaust pipe. For the heavy footed driver who squeals his tyres leaving work.
The "bottle tap" in the pub is another good one, lightly bang the base of your beer bottle onto the mouth of your victim's bottle, about 2 seconds later, beer everywhere!
If you have a new bartender, ask for a Guinness shandy, when they've cleaned up the spillage, tell them they should have put the beer/lemonade in first and watch it happen all over over again.
Mentoes in someone's Coke.
Unscrew the salt shaker and then redo a quarter turn.
Wax the striker on someones matches.
Tape frozen prawns to the top of someones exhaust muffler.