We had the same bingo boards as Erica - with the plastic windows that slide closed. They were great because there were no pieces/tokens to drop or lose or have sliding around when the car hit a bump.
A friend of mine gets a lot of games on line (google "free car bingo" or "free travel games") and then laminates them at her local printer. Her kids use dry erase markers that wipe off. Your kids have to be old enough to not get market on the seats, but you can also throw an old bed sheet over the seats for protection against markers and food.
You can also find the license plate lists where you cross off the states, but if you printed a map of the US and laminated it, the kids could actually learn some geography by finding the states. You could use markers, again, but I think you could also use the dot labels - get the garage sale removable labels, not the regular ones. There's a little more waste but a lot less marker mess.
My kid had a little tray table thing that hooked into the slot between the door and the window - as on airplanes, he used it as a "work" table as well as a snack table.
Do you have an organizer that hooks on the back of the front seats? They have slots and pockets similar to closet shoe organizers. Great for keeping things visible and neat.
We also love the big variety puzzle books you can find at any bookstore or magazine area of a supermarket - they have every type of puzzle so they are great for people who are good at different things.
I think a great resource is the public library. I have found some wonderful "old fashioned" games in books written in the 1950s and 1960s, before anyone had CD players and DVD units and smart phones, and there were only ever 2 radio stations in the area you were traveling through. There are great suggestions for things that will fascinate the kids - for the precise reason that they have never played them!
I like the suggestion below to do Mad Libs - they have been around for 50 years and are still a riot. And kids even learn what a "noun" and an "adverb" are - another bonus.