Are they driving? If so here are a few that work....

Fill their windshield washer bottle with milk. Cover the windshield with cereal. When they hit the windshield washer..cereal and milk. The milk will get real shitty once the car is running for a few minutes..lol

Your in ATL...there are tons of toads around. pay a local kid 10 bucks to find you 50 toads. Let them loose inside their car.

You can stick some old food inside their center console.. grumpy old men style..they will be remebering you for months after they go home.