L.N.
Perhaps you could throw some dried fruit in the mix--that would keep well. The main thing I wanted to caution you about is leaving plastic bottles with water in the car. If you believe any of the concern about plastic ingredients leaching into water, then plastic bottles that become warm and sit in a car are prime potential culprits. I'd recommend storing the water in something more inert like a large, stainless steel flask. In any case, refreshing the water every 2-3 days makes sense to me, and is relatively easy to do. You could also keep the water in a smallish cooler in the back, to keep it from getting warm. Good luck!