I am by no means a Disney expert, but in addition to water, we take:
Sunblock
Chapstick
Hats/visors
Autograph book and pen
Camera with extra battery and film or memory card
Cell phones and/or walkie talkies to keep in contact if our group splits up.
Stuff for kids, if you have them: diapers, extra clothes for young ones; wet wipes for any age (not just for diapers, after all), snacks, stuff to keep them occupied in long lines, like bubbles.
Hmm, I'm sure someone more experienced than I will have more ideas.
Have fun!