Another idea would be to have the aim of travelling as far as possible along a set road. That would gel well with the truck theme.
There's so much potential for this demo! I'd love to try out building a 3d environment with ramps, hills, hot dog carts to smash into, and other obstacles. Just the power up design space has plenty of room for growth, as does the variety of enemy vehicles you could encounter, especially if you start designing enemy types with abilities in mind and vice versa.