Got the law called on me by my friends grandma because of that once.. I like to use my old 3 hole punch, and fill the dash ducts with the "paper holes". Turn the air on full blast while the truck is off, and when he hops in and hits the switch, poof.
I did that to my brother's 4runner at his wedding. It was great. He tried to be sneaky and park it away from everything, and he ended up at the intersection of 2 of the busiest streets in town. Everyone honked and waved while it was being decorated.