Beef bullion in the shower head. Not exactly inventive, but who wants to go to work smelling like beef?
Vaseline on the doorknobs or the handle on the driver's side door on his car.
Seran wrap on the toilet seat or the doorways.
If your kitchen sink has one of those spray hoses with a lever that you squeeze in order to make the water come out, then wrap a rubber band around it to hold down the lever. When he turns on the faucet, he get sprayed with water.
Once again, not inventive. But it's unpleasant without going overboard.
Or, you could do what the Mythbusters did. Freeze a can of shaving cream/foam for 12-24 hours. Cut away the metal can and put the frozen cylinders of foam in his car. As they thaw, they'll expand and REALLY mess up the upholstery. Messy stuff.