Looking for vehicles with license plates from different states is one of the simplest ways to pass time in the car. Passengers can work together in teams, compete individually, or the game can be a group effort. It helps to print out a list of states beforehand so you can check ones off that have been spotted.
If it's a shorter trip, save the lists to keep the game going later. For longer travels, offer a prize to the person or team who spots the most states. Try keeping travel journal, so that players can log in the time and place where they caught a glimpse of each plate.
Another way to play is to use a blank U.S map to color in states as they grow further from the places you're driving through. Perhaps each color could be assigned a point value, with license plates from more distant spots earning players more points.