If you don't have a fridge, get a cheap styrofoam cooler and a block of ice or a frozen gallon jug of water.

I just read another poster saying to buy groceries on the way to parks for picnic lunches each day. We took a trip in September to GC, Zion & Bryce and had a cooler for snack foods and water. The block of ice or frozen gallon jug is a great idea - water from our bag of ice cubes leaked. :-O