This game is so simple, yet fun for the kids (and keeps them busy).
The goal is to spot as many license plates from as many states as possible. They can be on any type of vehicle. From cars to trucks, or boats to trailers. While we usually work as a team to find as many as we can, you could also play a variation where you keep track of who spotted the states and keep track of points. The competition or the introduction of a prize may raise the interest level of stubborn passengers (we all know them or are one).
I attached a sheet to help with this game. If you keep track of points, you could simply write initials next to the state or mark it out in different colors.
Note: This will most likely be harder to do if you don't travel through cities.