Rule #1: your kids good behavior in the car is limited to their attention span. Even if you have an over the seat DVD player, it will only last so long. So, here are more ideas.
Figure out how many state lines you're crossing, and head to the Dollar store.
Buy brown bags, and crappy toys.
Buy enough to put 2-3 toys in every bag per kid for every state line.
Bribe them to behave (while strapped in car seats - per federal law - for hours or even days on end) with said bags.
Also, the old stand-by's. "I Spy" was a WONDER to my 7 year old.