It also prevents the fingers from drying out and the skin from cracking. Drumming can use up a lot of water. Bring lots of liquid. Try stuff like Gatorade, watered down orange juice, sugar water, iced coffee, etc. Beer is nice at parties and you burn it up fast.
Sweet rum is traditional but forbidden in some parks.
I think plain water is the best bet. A few paper towels to clean up any bird presents or spills on the drum head and to wipe hands or a sweaty brow. Pencil and paper. An automatic pencil and graph paper come in handy to jot down rhythm ideas. To cover your drum in case of intermittent rain.