I remember travelling as a youngster and playing a game similar to the bingo game mentioned above. We were given a map of the US and each time a new license plate was spotted we got to color in that state.
"I spy" was a good game as it kept us occupied too and sometimes the craziest things would come of that game.
I agree that getting the kids out of the car to stretch and run around a little bit will go along way to avoiding the common road trip problems that can come up. One other thing that my folks did was to switch places with everyone in the car at every stop. This way everyone got a window seat, the front seat, or the middle for part of the journey. If there were more than one car, say with friends or family in the second car, we would even switch cars and people around for a few hours. It was just something to perk up the trip.
