badpacjac,
I would vote against the juice boxes in favor of the powdered mixes.

Even sealed, the juices can ferment. I have never seen a tube of kool-aid mix go bad.

I need to confess, I am a Dollar Store Junkie.

In my car I keep pop-tarts, granola bars, Rice Krispy Bars (the sealed ones) a 6 pack of water, and 20 or 30 individual drink mixes, all from the Dollar Store, totally maybe 10.00 per car. I feed the snacks to my kids the week I restock the car.

I keep a few longer term items, Clif Bars, 2 mainstay bars, and some really disgusting hardtack, but most other stuff gets swapped every few months.