How about different choice in rims?
And since we're on the car subject why not turn the stars into Honda emblems and the barrels into Nissan symbols or something like that. Also, to make things VERY interesting, offer different SIZE rims and Tires, and different thread patterns. That way you can create some variation in speeds, acceleration, and air time(bigger weighs more!) and even create some leveling up if you plan to make your next one with many more levels.
I'm just turning your simple and fun game into a long project :p have fun!