Jan in VA mentioned a VW or VW bus and I think either or both of those is a great idea. Maybe put a flower design on the door or just use big flower fabric? I know there are quilt patterns out there for both those vehicles so maybe you even could look at them and design your own.
Well, they turn out to be easy to find so here you go:
Here's the bus one:
http://www.ebay.com/itm/GROOVY-NEW-P...item5d41a23ff2
And here is the car:
http://www.ebay.com/itm/GROOVY-NEW-P...item5d41a24017
This should be a fun quilt to make!