I always carry in my fanny pack the following:
small bottle of saline (for contacts)
money/credit cards/admission media/hotel key
sun glasses (I like to put them away when we go inside)
small fan (bought at WDW Epcot Land gift shop) - it's approx. 4" tall and 2" around
precut moleskin, plus some long strips of moleskin with a pair of folding scissors
Palm pilot - has menues, schedules, park maps, things to do in each park, etc.
Travel container that has just a few of the following in it (I replenish the container if we use any during the day):
Advil
Tylenol
Excederin (has caffeine in it and is a great pick me up at lunch)
Ammodium
Tagamet
Small bottle of hand disinfectant
We also bought rain coats at Epcot Norway the last time we were there; I have some elastic straps that seal with velcro; my plan is to roll each rain coat into a long tube shape about 12" long, and use the straps to go attach the rain coats around the belt of the fanny pack, and we'll carry them at our backs. Should also make a nice lumbar support when we sit. We've tried ponchos and we HATE them - that's why we bought the rain coats!
I also put a small caribiner/clip on my fanny pack (on the waist belt); when we go inside, I take my hat off and clip it to the caribiner, and don't have to worry about losing it or forgetting it after a show.