Billboards and road signs can be a source of educational entertainment, and a way for children to practice their ABCs. As you pass signs along the road, try to find each letter of the alphabet in order. The caveat is that you can only get one letter from each sign you drive by.
When you see the letter, call it out. For example, you'd say, "I see an 'A' on 'Dairy farm this exit'." Nobody else in the car can call out a letter using that sign, even if it also said, "Fresh butter." You'd have to get the "b" from a different billboard.
This is a game you can play all together as a family, or you can pit family members against each other. To sweeten the pot, you can guess how long it will take you to get through the alphabet. Provide prizes or rewards for the winners.