Scary ice-cream truck would be cool, but what I think would also be funny would be a UPS/FedEx/DHL/etc. truck that flings packages out of it, like they did on that show "Monster Garage" (if anybody remembers that) or just drops them out the back or through a hole in the floor.
Or
Paint it with some "Vote for (insert idiot political guy here) showing him and his happy family on the side" and put those huge old school loudspeakers on top with a sound system. And the sound system could just sound like the adults on Charlie Brown cartoons. Wwaaa waaa Woo wwaa wwwaaa wa. Cause all politicians sound like that if you listen to them long enough
Anyway looking forward to seeing what you really did with it...