The things I keep in my fanny pack:
small first aid kit/meds--contains things to clean and cover a scraped knee and blisters and stuff to fix headaches, tummy aches, heartburn, allergies and asthma
sun screen--small tube
chap stick
digi camera and a zip lock bag to keep it in when riding water rides
small battery fan---I buy the really small ones that are $1 at
Walmart. They worled fine for us. If we had gone during a hotter part of the year I would have bought larger fans that squirt water.
breath mints--not a must have, but I like them. My dh is a gum chewing guy.
Diaper bag:
diapers
wipes
change of clothes for baby
change of shorts and undies for big kid
large zip lock bag for soiled/wet clothes
cereal in single serving bowl/box
Other things I'm so glad we had:
glow sticks for the kids
Brita Fill-n-Go water bottles--these have been discontinued, so stores are selling the last of them now. I paid $4.50 for a bottle yesterday at WalMart. Bottled water in the parks will cost you a pretty penny over the course of a vacation. I'm glad we had them when we went last October so I picked up two more yesterday.
Visor--I'm not much of a hat person, but I loved my visor. I wore it everyday.
I made sure everything we brought in the parks could fit in the fanny pack and small diaper bag that way we didn't have to leave anything in the stroller while riding rides. And it was all easy to carry.
Do not forget the battery charger. We had to charge the video camera every night and we kept an extra set of batteries on the charger for the digi camera. We changed out the digi cam batteries every morning.
Also, a helpful packing tip: group like items in ziplock baggies...large ziplock baggies. Even in your carry on. It makes searches so much faster.
Ok, that's all I can think of right now.