Used this idea for years while camping in Baja...

Buy water in plastic containers from your local store... either 1 gallon or the 2.5 gallon containers. Take a bit of water out (for expansion) and then freeze the container.

Two benefits from this... one, as the ice melts, it doesn't turn into a giant puddle in the bottom of your ice box; two when it melts, you have more drinking water. (ice cold drinking water)

Another idea... get a long term "5 day" ice chest... and freeze anything you put in there first... open this as little as possible... it will last over 5 days. This even worked in Baja heat (kept that ice chest in the shade at all times)

Gotta second the idea of the solar shower... they work better than you might think... but use it in the warmest part of the day... don't wait "until later" for that rinse off shower.