You could always cover their windshield with crushed Cheerios and fill the washer bottle with milk. When they hit the wipers to clean off the mess....Instant breakfast on the go. Very hard to clean out the milk after a quick drive to work.
Superglue the Zipper on every pair of their pants in the down position.
Wire the brake pedal to their horn trigger. (very easy for any car audio installer, even John)
The dead fish trick from Grumpy Old Men. I recommend hiding it in the console. It stinks way worse after being warmed by the transmission.
