Yep, parks. Get off the highway into the small towns and find parks with playgrounds and let the kids burn off some energy as often as possible.
Play traveling games - I Spy, treasure hunt (write a list of things to spot 1st spotter gets credit, black cow, red car, a statue, a bridge etc), snacks keep the busy. I give them packaged snacks because fighting the wrappers takes up some of their time.
The DVD player is great when you need a bit of peace and quiet but I try to limit it because I don't like my children wasting the experience of a journey staring at the back of the seat.