On our trip we (of course) played the license plate game. We just jotted all the different ones we saw down on a scrap of paper. Which we then lost, but it was fairly easy to recreate, surprisingly. When we got back I had the boys color in the states we saw (I meant to print a map that included Canada, oops).
We may have also seen Tennessee or Kentucky, and one or two more Canadian provinces. TONS of Californias, Nebraskas, and Missouris for some reason.
National Parks are SO the place to play this game, it was amazing the variety we saw just four hours from our home! The guy with the Alaska plates may have thought I was a big dork because I was pretty excited to see his car!