1) Pack light - it gets to be a drag carrying stuff around the parks all day.
2) Disposable rain ponchos.
3) A hand towel or two. (great for wiping yourself off, or the seats on a ride after it has just rained or someone has spilled).
4) Sunblock, sunglasses and a cap or hat (of which WDW sells some really nice ones!)
5) Any reservation or confirmation #'s for dining or tours.
6) Tickets, and petty cash or room key if using to charge.
7) Cell phones or walkie-talkies to remain in touch with other members of your party in case you get separated.
8) Single size or sample packets of: Tylenol, Aspirin, Pepto Bismol tablets, Immodium and Benadryl. Or, a small pillbox with a couple of each of these in it.
9) Optional: change of socks. One thing I can't abide by is walking around with wet socks (either because of sweat or wet ride). But maybe this doesn't bother some people.
10) Travel or sample size of baby wipes. They're handy for just about anything.
I can't really think of anything else off the cuff, but this should give you a good start, and I don't think any of it is too cumbersome or heavy. A water bottle is nice, but schlepping liquids can get awfully heavy, and while I hate to spend $$, I would personally just buy a bottle of water at the park. And, since this is your first time, have a blast!
