We always bake a batch or two of HH muffins, ingredients for PB&J, fresh fruit, bean burritos, trail mix, the granola bars p. 23 in
Everyday Happy Herbivore
. If we're stopping overnight at a hotel, soup. Though you can heat soups in most gas stations. I've also heard great things about
McDougall's Right Food
soup mixes. I think you just add water.