My husband and I love our camels. We use them when we mountain bike, but when it's hot and humid, I hate having that thing on my back. I think in the parks, especially in August, I'd rather carry a regular backpack, or one with insulation in it instead of a camel. The regular backpack won't fit as snuggly to your back, so you may be cooler because of that.
As for keeping the bladder cool, ice can certainly be added throughout the day, so I don't think that having a warm drink on your back would become an issue as much as just having a hot and snug pack on.