Bring a cooler and pack it with nuts, raisins, fruit, and veggies such as baby carrots. Don't forget water and juice; they're healthier than soda, and they'll lead to fewer bathroom trips than caffeinated drinks do.
Instead of the usual fast-food pit stops for lunch, go to a supermarket and pick up salads, cold cuts, cheese, and bread and have a picnic. Always keep paper plates, plastic utensils, and one real knife on hand, as well as a blanket or a sheet you can spread on the beach or any patch of grass.
Portion control with snacks is essential. Measure out an appropriate amount of Goldfish or pretzels and put it in a Ziploc bag. Otherwise, kids might eat too much--and probably make a mess in the process.
Make exercise part of the experience. Bring bicycles for everybody and find a place to go for a ride, and sightsee on foot every day. Consider staying at a hotel with a pool. Kids love them, and splashing around in a pool is much better than sitting around watching television or playing video games.