Books on CD are a nice ride eater. Sometimes we find a book/CD or two at Cracker Barrel and then drop it off at another one when we are done.
Interactive games we've done:
Name something you see beginning with the next letter in the alphabet (armadillo, boat, car...)
Storytelling handoff... each person contributes a line until the story ends. Can be extra fun with a tape recorder for later.
DS6 will sometimes just watch the scenery and think. But we've learned never to give a time estimate in case he wants to figure out the seconds and count them as we go..... argh
