We did this last summer and we're about to do it again this summer.
Our daughter was almost 2 then, and she's almost 3 now. We ate a lot of avocado/canned refried bean sandwiches, we ate a lot of fresh raw foods, subsisted on watermelon and pineapples quite a lot, and oranges, (we were in S. Florida). That was a cheap, calorically dense/filling and yummy way to eat. We also grilled corn, zucchini, onion, a decent bit, and ate salads and snacked on celery, cucumber, and ate whole delicious tomatoes mono meal style... we drank orange juice if we didn't have a lot of fruit, and sometimes got mangoes or whatever else was on sale, but dates and making our own simple trail mixes lasted really long and were quite cheap for the calories/energy/fulfillment, and we didn't eat that much of it, so we were able to have it for emergencies. We stayed away from energy bars/granola bars etc. except once when we got a box on a huge discount, and I think we didn't even eat them all, we didn't like them too much, it as an odwalla flavor. We did get rice cakes too, and my daughter ate them quite a bit while driving in the car... a low mess food that was quite cheap and filling.