Young Children
Young children can participate in counting or color games. Color games can include counting how many cars of a certain color they see in a certain time period. Counting cows or sheep can entertain a young child if the countryside has enough for them to stay interested. Other counting options are mailboxes. If you are driving through a town they can start counting red or green traffic lights. Smaller towns may only have stop signs but these can also be counted.
Alphabet Game
For children who have mastered the alphabet, this game will keep them occupied. The players decide on one letter from the alphabet to start the game. If the letter is "T" for example, the first person to spot three things that starts with the letter "T" wins the first round of play. The winner of each round gets to select the next letter for the second round of play.
Geography Game
Older children and adults can participate in the geography game. Start the game with a city, state or country from anywhere in the world. Florida for example, ends with the letter "a." The next player must think of a city, state or country that starts with the letter "a. "If the player says "Arkansas," then the next player must use the letter "s" because "s" was the last letter in Arkansas. Younger players who are not as knowledgeable may need help with spelling or may need hints from the older players. Players are eliminated when they cannot think of a city, state or country. This rule can be changed to allow a player to pass on a turn and still play in the next round. This game doesn't need to end with a winner, just play until the game gets boring.
License Plate Game
Making silly sentences from a license plate is always entertaining. If the license is 234 LMC, it can become Love My Cat or Little Mice Cry. Every player makes up a phrase because there is no need to keep score or have a winner. Even adults can be entertained with this quick game.
Start keeping track of cars that have out-of-state license plates. The first person to spot a different license wins the round.