Here’s a thread full of useful tips at rec.backcountry.

On Sunday I did my usual trick of throwing a tray of ice cubes into the hydration bladder. While I was fumbling through the fridge I remembered the fresh melon Melissa had cut up the day before and left in cubes. I tossed a bunch of them into one of those Rubbermaid sandwich boxes and made sure to pack it next to the hydration sleeve in my pack. Then I stuffed my inflatable sit pad on top of it for extra insulation. It made for a heavenly snack after a few hours of hiking in the hot sun, and the pad kept my water cold for more than five hours.