A local car wash. Get a local car wash to sponsor the event or ask a convenient shopping plaza if you can monopolize their parking lot.
Most businesses are happy to oblige if their name is prominently displayed on the flyer.
Make a flyer up ahead of time to entice them. Send home flyers asking parents, friends and neighbors to come on down and get every car they own washed. Get students to put up signs on every main street corner. This, however, requires access to an electrical outlet. You pay only shipping and handling.