The Banana Game
If you're looking for a game that doesn't require a lot of thought, try the Banana Game. Keen observation is the only skill required. The first person who spots a passing yellow vehicle gets points. You can award points based on the size or make of the car. For instance, a yellow school bus might be worth five points, and a yellow sports car might be worth two.
Another way to heighten the competition is to put a limit on the time you have to spot bananas. For example, the winner of the game could be the person who spots the most bananas in 30 minutes. Or the winner could be the player who sees the largest cumulative number of yellow vehicles during your entire road trip.