DH and I always bring a bottle of water with us. There are ample water fountains throughout the parks to refill your bottles during your park visit.
I think we refilled our 20 oz. bottles maybe 10 - 12 times each on a recent Sunday in Epcot.
Some people bring Crystal lite or other powdered beverage to flavor the water. When your hot, anything cold tastes great. I didn't mind their water one bit!
We also bring gum, candy, and other small snacks, like maybe baggies of raisins/peanuts/M&M's.
Madison