I'd start out with more than 4 gallons of water and would add Gatorade powder, NUUN tablets or some such to my kit.

On muggy days hiking in the Shenandoahs (Virginia), I've consumed a gallon of water and not had to pee, such was the sweating.

I'd add a whistle and a Bic lighter, cotton balls, flammable lip balm and a stainless cup.

And whip by a grocery for some food (cold oranges would hit the spot on a hot day).

So that I could carry ice, I'd also seriously consider buying a decent cooler (Coleman X-treme) and saving the box it came in so I could ship it, with some of my gear, back home via UPS or Fed EX.

For the last twenty years, I've been in the habit of Fed Ex-ing to my destination my luggage (insured) rather than risking the airline baggage handling system. That way I take my hiking boots, poles and other outdoor gear that I consider essential.

Makes the airport process a simpler and faster, too.

Are you planning on venturing any distance from the car?