Have you done route 66 in the midwest? It's not as good or intact as in the west, but there are still plenty of old towns to visit with kitchy route 66 stuff. Or maybe something related to the oregon trail? It would be a long drive. There's Chimney Rock in Nebraska and wagon ruts in Wyoming.

How about a BBQ tour? In my opinion the Kansas City BBQ is best, but you could check out the BBQ scene in St. Louis and Memphis.