this one is lifted from Mario Kart...
attach 3 balloons to the car, maybe one on each side and one on the rear. the object would be to pop the other's balloons. when all 3 of your balloons are popped you're out. last car with a balloon wins.
the targets don't need to be balloons, maybe some dish washing sponges that can be velcro'd to the body as long as they can be knocked off on impact.