A good way to keep everyone in the car entertained and engaged is to play a memory game such as The Grocery Game. To begin, the first player names an object available at a grocery store that starts with the letter A. The next player has to repeat what the first player said and then add another grocery item that starts with a B. For example, if player one says "apples," player two would repeat "apples" and then might add "bananas." If you forget a grocery item, you're out, and the game continues until the player with the best memory wins.
If groceries aren't your thing, you can play variations of this game with anything from animals to sports to people's names.