I bring entirely too much to the park, but I don't care. I am prepared!
I carry a Coach backpack that holds a couple bottles of water, a little first aid kit (just band aids, blister block strips and neosporin, I can't perform impromptu surgery) antibacterial wet wipes, my phone, a wristlet with my ID, cards, money, a compact (gotta be pretty!), hair ties, chap stick, nail clippers, a tiny nail file ( I have to take care of nail issues ASAP, it drives me crazy to have a ragged one)a pen, a spray bottle of sunscreen and a folded up poncho in the bottom. I'm ready for anything!
The backpack isn't large, I'm used to carrying some pretty cavernous handbags, so a backpack in a theme park is no issue.