Congrats on the Disney Vacation. My wife and I (and her friends from school) spent 6 nights last June and had the time of our lives. Here are my suggestions on gear to bring. Not all of it needs to be carried every day:

1) small flashlight (like a photon II) - we were surprised at how dark the parks get after the sun sets, particularly in the back half of the magic kingdom. I had the one that has the flashing strobe. At night I could hold it up and the group could find me easily.
2) sunscreen (probably an obvious item)
3) I carried my Leatherman Squirt in my pocket and it was never checked. The scissors were useful.
4) In the summer I would carry the $.99 disposable ponchos (have a few extras in the suitcase in the room). If it gets lost, its no big deal. We left ours in the room when we checked out in case the staff wanted them. Buying a Mickey Poncho in the park will probably be over $6.00 each.
5) Moleskin - somebody in the group always wears shoes that cause blisters.
6) Packets of Crystal Light/Propel that are used with .5 liter bottles. It seemed more refreshing than just water.
7) If you plan on washing clothes during your visit, bring the sample size detergent boxes (wal-mart sells them cheap. Disney charges three times as much in the laundramat.)
8) Several rolls of quarters (left in suitcases). It's sometimes hard to find change nearby.
9) Bottle of purell - who knows what your hands have touched? Purell and a napkin can clean up up if you can't find a restroom.
10) Bandanna - I would wet mine and wear it around the neck to cool off. At night I would wet it and put it in the in-room fridge so it would start off cold.
11) Hats - I wore my Tilly sunhat. The brim and construction kept me cooler than those who went hatless.
12) Earplugs. Some of the music/sound effects on some rides were too loud for my ears. Besides, who really wants to listen to the lyrics of "It's a small world..." more than once?
13) Sewing kit with extra buttons and safety pins.

Other hints:
1) Stay away from caffinated drinks and sugary sodas. Water is far better.
2) Get up early and go to the park that has the special morning hours. We would then leave after lunch, go back to the room and take a nap and shower. A fresh shirt, socks and shoes and we would leave around 4 to go back to the parks. We seemed to have extra energy. With the park-hopper pass we often would go to a different park at night.

Hope you have a great time. I'm interested in learning from your experiences when you get back.