Pack a Cooler
Start with water and other drinks in there, plus some snacks. When you're driving cross-country, you can easily drive dozens or more miles between cities and come across closed rest areas and tiny towns without restaurants. It's ultra-convenient to just reach behind you in the car to grab a nibble and a sip.
Once en route, coolers also help save money on dining costs. Stop at local grocery stores to get muffins, fruit and orange juice for breakfast on the go, and sandwich fixings and chips for an easy lunch at a roadside stop. If you're more ambitious, toss a spatula, tongs, matches and charcoal into your car. Then you can buy hamburgers and hot dogs (and whatever else you like to barbecue) on the road to cook out at public grills in parks and rest stops.
When you do find a supermarket, grab healthy snacks like fruit, nuts, low-fat string cheese and yogurt. It's not always easy to healthfully on the road, but fast food quickly gets old -- and it can do a number on your tummy while in the car.