We are flexible based on what we're doing. We are a family of four: me, DH, DS8, and DS10.
Everyone wears their own hat and pin lanyard or bag.
DH and I carry ID, APs, credit card, cash, cell phones on our persons.
DH can slip the camera into his pocket.
If we are going to the park for only a few hours and likely going to eat some park food, this is it! No other stuff. We only do this occasionally though. I guess we just feel more secure lugging around all the stuff listed below.
We fill one backpack that DH and I switch off carrying. Depending on weather, where we are based, and what activities we have planned for the day, we will carry the following additional items inside:
3 or 4 water bottles (can always refill)
4 apples and 4-8 crunchy snacks (cracker thing, granola bar, peanuts, potato chips, something like that), sometimes 1 or 2 Crystal Lite flavor packets for the kids
sunscreen (always, always, no matter the season. DS8 has red hair and really fair skin; and the rest of us look pretty pale too

)
small tube each Motrin and Tylenol
travel-size Tums
chapstick
a few band-aids of various sizes
Kleenex
leftover
Disneyland napkins from previous trips
hand-sanitizer. We often don't bother using it, but if I want to get the kids' hands "wet" when we're not near water, it's a quick solution.
couple empty baggies for putting things in during water rides
empty garbage bag for covering backpack during Grizzly River Run
baggie with quarters, dimes, nickels, and pennies for smashing machines
One of the following categories:
sweatshirt (preferably hooded) for each person (only in non-summer months, otherwise I tell everyone it's really not that cold even at 10 PM, even if it is

)
poncho for each person in the group only if rain is almost certain, and then everyone has to wear the sweatshirts all day or tie them around waists or leave them behind
towel or blanket for the ground or to wrap up in, only if there is space since we're not carrying the clothing items. And we usually get this wrong, carrying it only on days that don't turn out cold and that we don't stay for a night-time show and then not having it with us when we stay for the 2nd Fantasmic the next night and get all damp and chilled.
We also have a reusable, packable Disney shopping bag clipped to the backpack's zipper tab which I've only remembered to use once.