On my last few trips - my WATER BOTTLE. I have one of the Brita Fill and Go type with a filter in it to take the metalic taste out of WDW water, but any type of water bottle that you can carry and refill can be a lifesaver in the parks, even during mild weather.
Drinking fountains are somewhat scarce in the parks these days, possibly to encourage the purchase of bottled water, so carrying your own supply with you is a big help. Not only will it keep you cooler and hydrated in the hot weather, but even when it's not so hot you'l still get thirsty once in a while in those long attraction lines, and having some clean water to rinse your hands or wipe your face with can also be very handy.