Create a Scavenger Hunt
Scavenger hunts are great ways to keep your children involved in the traveling aspect of your journey. While you're preparing for your trip, create a list of items you think you'll see along the way. It can be a list of random objects, or it could have a theme like types of vehicles, farm animals or road signs. Make copies for all the passengers in the car, and as each person sees an item, he calls it out, another person in the car verifies it and he crosses it off his list. The first one to cross off the entire list wins a small prize.
Another variation of this involves creating a bingo card for each person. Whoever creates a bingo on their card wins. You can find ready-made printable cards on the Internet, or you can make your own.