MP3 & DVD players are great to have on hand but they don't allow for as much family interaction. Our road trips always had conversations going back & forth, playing the Alphabet game, naming the states, the story roundabout (where one person started a story with "One day..." says a few sentences, then another would pick up the story thread...this was one game the driver could play as well
). We'd stop every 2 hours for 10 minutes to let everyone stretch their legs, grab waters, etc.
One of our favorite Road Trips was when I gave each child 3 tickets, one each for Lunch, Dinner, and Stop & Explore. At any time, any one of them could spot something on the road they wanted to check out & we'd stop, no questions asked. The kids loved it because it gave them some decision power & they were all much more invested in the trip. It made for a memorable, fun, & free-wheeling Road Trip.
Here are some other links that I found which offer other ideas & practical information: ActivitiesforKids, Traveling with Toddlers, Kidzworld.