car bingo - you can get the boards on Amazon, ebay, etc.
music - make mix tapes (mp3 playlists) of your favorite classics, their favorites, etc. At that age, they are usually happy to listen to anything that can be easily sung along with. Try Arrogant Worms - they are very popular with that age group and much of their stuff is actually entertaining for adults too.
planning - bring guide books, sightseeing maps, etc., and have them plan out the next days' events. Camping cookbooks - let them plan meals and help with on-the-road grocery shopping.
digital cameras - they can review the events of the prior day via the camera while driving down the road to the next memory making spot. Set up contests with them (e.g., take a photo of wood - the most creative shot wins) and the winner gets to pick a restaurant or a sightseeing stop, etc.
story telling - make up stories, pick up books on local ghost stories at various stops (California coastline has lots of those), books on short stories about the area, etc. (example, Journey to the Brink of Doom by T.W. Kriner for tales of Niagara Falls and folks who went over). Have some on a kindle, but check for local printed works at various stops.
With my nieces, the one thing I learned was to be silly with them. Give in to your inner child and let it lead you with the kids - they'll love seeing Grandma and Grandpa getting silly, singing funny songs, etc.