How about individual peanut butter packets? That's another popular kid-food my wife packs, and is easier/less messy to eat with the squeeze packs... The Justin's brand are gluten-free and all natural which is important to our kids, but probably means they don't keep as long. Since we use/pack these in lunches already, would be easy for us to rotate them into a car pack.

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What jerky do you use? What's the longest you've kept it in the car and still eaten it?