We usually take a Camelbak for DH, DS3 and me. We use the water bladder part to hold cold water w/ice so we can stay hydrated, even while waiting on line.
sunscreen packets or a small bottle
lip gloss/chapstick with sunscreen
antibacterial hand wipes
tissues (in case our allergies flare up)
sunglasses/hats
nail file
money/cc/traveller's checks
some hard candy and gum
snacks like granola bars
little battery operated light up fan (to keep cool and entertain DS)
glow sticks/necklaces/bracelets
salt/vinegar packets (for french fries-most places only have salt)
cameras and batteries (all in ziplocs)
bandaids
single serve drink mixes for bottled water
tide to go pen
cortaid (I get a lot of bug bites)
ponchos
quarters and pennies, if you want to do pressed pennies
ADR list with numbers (also stored in my cell phone)
cell phone (also in a ziploc)
park maps-one from each park so we can plan while waiting on line
email addresses (if we're going to EPCOT so we can send virtual postcards in Innoventions)
change of clothes for DS
I think that's about it
. Honestly, it sounds like a lot, but it really isn't. Most of the items are really small/lightweight.