We take a Camelbak backpack to haul our stuff. They come in various sizes and have a water bladder built in so you don't have to carry water bottles. We just pull out the bladder and fill it partway with bottled water at night, then put it in the freezer of the hotel fridge (works with pretty much any size fridge freezer) to make a frozen block. In the morning we top it off with more cold filtered water and we have cold water to drink for most of the day.
We also carry baby wipes
antibacterial wipes
sunglasses/hats for everyone
sunscreen (mini bottle or packets)
gum/mints
snacks
tissues
sippy cup
misty fans
light up fans/glow sticks
camera and extra batteries
Tide To Go Pen
extra outfit
lip gloss w/SPF
ponchos
cell phone (with ADR's saved in the memo section)
restaurant lists (I'm a picky eater so I search through All Ears' menu section before we leave and type up a list of places that have food I like)
ziploc bags (to keep things dry, or for wet clothes/restaurant leftovers)
email addresses (so I can send video postcards from Innovations @ Epcot)
dental floss
quarters and pennies (for pressed pennies)
park maps
nail file
neosporin
jackets (depending on the weather)
lollipops and little toys to keep DS from getting bored
Wow, it sounds like a lot, but so much of it is tiny that it really doesn't take up much room.