I see you have tons of great suggestions already, but I have a few too...A box of tin foil, I know it is not a very "green" idea but in a pinch it works. We were traveling just before thanksgiving, we all made headbans with foil feathers in them, birds, lumps of potatoes etc.
Plan to stop somewhere fun, not just a play ground, but something you will be driving past that is special to that area. We were driving from Orlando to St. Louis, we had never seen the change of leaves wo we we played for a few hoursin the leaves in Chattanooga, took the rail way for a tour. It gave the girls (then 2 and 5) something to look forward to, plus they burned tons of energy. That 2 hour "lay over" was one of the best parts of our vacation. We also gave them their own disposable cameras.
We also printed out I spy pictures from the computer, they got a sticker or silly stamp every time they found one.
We had books on CD. We also took a few silly chapter books, Junie B Jones is always a hit for my girls, the adults took turns reading and we changed our voice for each character. Lot's of Laughs.
For us vacation starts the minute we step out the door, make the most fun out of it and don't sweat the small stuff. Have fun and enjoy the adventure and the memories in making...