As a single adult, I take a small backpack with the following:
1) A tupperware container made for holding a sandwich with PB crackers, granola bar, & fruit snacks inside. I'm a cheapskate and rarely buy food in the parks
The container keeps the snacks from getting crushed inside the bag, and is small/flat enough to not be bulky.
2) My
PackIt with a Buddy Fruit and/or cheese stick inside
3) An umbrella, if it looks like it might rain
4) Sunscreen
5) My
S'well bottle, in a side pocket
6) If I'm going to Epcot, a small empty water bottle for filling with Kinley
And the following items from my purse, which I always have with me anyway:
7) Ibuprofen
8) Lip balm
9) Gum
10) Hand sanitizer
11) Hand lotion
12) Keys
13) Wallet
14) Phone
Sunglasses are always on my head, and if my hair's not in a ponytail already there's a holder on my wrist
That might sound like a lot, but it's actually not bad at all. My backpack is small and light, but has enough extra room in it for snacks, etc. for friends I may be with and any merch I might pick up.
My friends who have kids bring the usual diaper bag, water bottles/snacks, ponchos, etc. and stash everything in the bottom of the stroller. When I go with them, I sometimes leave the backpack at home and just take my crossbody purse; I can put my snacks & water bottle in the stroller.