Most of these instruments could be automatic and be played by
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.
(see idea of this name) -- wind instruments with
facing in the direction of travel, with serialized mechanical
scores incorporated into their structure.
Caliope-like instruments working on wind or the movement of the
Obligatory megaphone for sing-along.
This could be a more-than-one-weekend project.