You know, you're given a budget, and you have to build a race car, and I'm not just talking put new parts in it. You have to build it from the ground up, frame and body and everything.
And it doesn't have to be just cars, it could be bicycles, or airplanes, or moon rovers, or tanks, or whatever.
Or better yet, maybe just give a task for the vehicle to do (i.e. pick up and carry an object from point A to point B within 2 minutes), and let the player make any kind of vehicle he wants, and then drive it himself to complete the objective.