To keep the kids happy in the car, a mystery bag to be given to them about each hour. They get a map with each city marked, and they have to follow the route and ask for the bag when we get to that city. Bags are filled with snacks and small toys and games.
Road Trip Snack Boxes for Kids - I found the boxes in the craft section at Wal-Mart and included a variety of salty and sweet treats. Then on each leg of our trip (divided up into 1-2 hour increments), they could choose one or two things to eat. Very convenient on us as parents and the kids love it, too.