Help your child learn to watch for road signs, and follow along. This teaches important skills, and helps fill the time. Buy books on tape. You can either pop one into the car stereo for the whole family to enjoy, or give your child a portable stereo with headphones to listen alone. Play the license plate game.
Older kids can even try to write down the capital of each state as they go.
Look for numbers, colors or letters. Tell tall tales. Have one family member begin telling a made-up story aloud, and stop at a crucial point. Someone else has to take up the tale, and the story goes around until everyone has a turn. Make room for naps.