Two words: Wet Vest. I have one, keep it wet and frozen in the freezer (so the water doesn't get "stinky" in the gallon plastic bag.)

And water. Lots. of. water.

In addition to that, I take on long day trips where the temps go into the 90s some quart baggies of cut up fresh fruit in with the frozen water bottle and wet vest, which goes in a soft-side carrier.

I take baggies of
grapes
watermelon
canteloupe
honeydew
strawberries

Snacking on these really keeps you hydrated, and adds necessary micro nutrients and non-sugary-calories to your day.