i just got back from disney in florida, three things i was wicked glad to have were a brita water bottle (the filter got rid of the taste from the water fountains), sunblock, and a little baggie with a small bottle of gold bond and a spare pair of socks (for the days i wore sneakers)
i was also happy with the ems packable pack (
http://www.ems.com/product/index.jsp?pr ... 2.11373430) the bottles could be wedged into the side pouch, and the $15 reusable mug from the hotel could be clipped into the hook on the inside (bring a mini-carabiner) and not just form a pile of stuff at the bottom. its a light pack, has a sternum strap for comfort, and disappears into luggage on the way there and back. it also fits into the tiny webbing things they have in front of you, for you to store your stuff on rides. (as long as you don't pack it super full)