Get a road map for each child. At home have a basic lesson in map reading and have the kids mark the routes you will be traveling. On the road, show how highway signs relate to the map. Make a game of estimating how long it will take to reach a certain town. Let kids take turns giving directions.
Older children can also help keep track of mileage, miles per gallon, and trip expenses.
Have children take turns pretending to be a talk show host. They can use a tape recorder and have other passengers discuss what impressed them most about attractions or pretend to be historical characters related to the sites. The tape makes a nice trip souvenir. The type will vary with the skills of the children. Take along a collection of family stories for long stretches of highway.