Watch for Plates
License plates can be the basis of a number of fun car games. Before you start your road trip, ask each person in the car how many different state (or province) license plates you'll see along the way. Write down everyone's guess and keep a running list of all the states and provinces you see. At the end of the trip, the person with the guess closest to the actual number wins a prize. If you're doing a multi-day trip, you can even play this game on a day-to-day basis.
Kids can also mark off all the state license plates they see on their maps. If you have older children with really keen eyes, they can track the different specialty plates you see along the way, like those promoting education or animal rights.