Back when we did Disney on a Dime type trips, we didn't even eat meals in the parks. In addition to the other things previous posters have mentioned, we got those tuna-salad-in-a-can-and-cracker type heat stable foods and took those into the park for lunch (then we'd do fast food for dinner since we were staying off-site). They're surprisingly decent and the protein is nice to tide you over.
For our plane rides and such, my favorite heat stable food is still spam musubis (salted rice + spam + nori wrap). Google it. It may not be for everybody, you have to like spam which seems to mainly be people from certain regions, but packs and travels really well. Hard to make right before you go into the parks, but I often make several for each person before we go on the trip, and we have one or two on the plane, then one or two the first day in the parks.