We bring most of our own food when we're in the parks, here's what's in our backpacks every day:
I have a small softside cooler that just holds two bottles of water. I freeze the bottles the night before and then add string cheese to this same pack, we bring crystal light to cover the Florida water taste.
I make my own mix of raw almonds, salted roasted cashews, and m&m's if it's not going to be too hot. For hot days, I leave out the m&m's.
I also bring Dried Fruit, cookies (love the pringles can idea!), licorice, goldfish (cheddar or pretzel), rice crispy treats. My sandwich is pb and jam so it doesn't need to stay cold.
We do a ton of travel that requires some sort of easily portable and non-perishable foods - my two go-to's are nuts (usually raw almonds so I keep the salt down) and dried fruit. You can almost live on these two things - good source of protein to curb your hunger and natural sugar for a short term boost. Costco carries the best dried fruit mix, it's their Kirkland brand Dried Mangoes and Berries.
We've never had good luck with bananas or muffins, no matter how carefully I pack them.