Why not bust out some general automotive trivia? Nothing generalized (e.g all about ford, chevy, etc) just a broad range of question about random production cars, hell, even music from certain eras (like questions about what Beach Boys song talks about a girl wishing her boyfriend luck when he drag races today?). My husband says you guys could have a "fashion contest" where you have all the car show people dress in clothes from the 50s, then judge and award accordingly. What about valve cover races? I guess it all depends on how far you wanna go with it.
P.S. If your cruise-ins are anything like the ones around here, you could always blindfold people and have them play "Pin the Whale-Tail on the Nissan". <---my husband came up with that one.