One-Car-Band

Most of these instruments could be automatic and be played by the movement of the car:

Drums facing down toward the road with hard rubber tipped drum stick on a hinge so that it bounces between the road and drum. Clock-like gears could be used to create different rhythms.

Truck-flutes (see idea of this name) -- wind instruments with funnels facing in the direction of travel, with serialized mechanical Gatling scores incorporated into their structure.

Caliope-like instruments working on wind or the movement of the car.

Obligatory megaphone for sing-along.

This could be a more-than-one-weekend project.