I wear a medium-sized crossbody purse (one that has a full zipper across the top, so nothing falls out if I'm on the Rock 'n' Rollercoaster!), and it holds ID and credit cards, KTTW cards, camera, phone, first aid mini-kit with bandaids and Tylenol, sanitizing wipes for our hands at mealtime, sunglasses, and DH's inhaler. Crossbody bags are great because so long as you keep the load light, it's like clothing -- you just put it on and don't have to think about it all day. I leave it right on during attractions -- depending on the restraint system, I just adjust it toward the front so the purse hangs between my legs (ooh, so classy!) or tuck it next to me. I have a Very Bradley hipster, as well as an off-brand fabric crossbody bag I got at Target, and I use them interchangeably.
As for the waterproof factor, I keep a ziploc bag inside the purse for my camera and phone, in case of wet weather or wet rides. I'd rather carry a "breathable" fabric bag since it's going to be against my body all day, and as a result I find I don't get sweaty where the bag hits. My DH once made the mistake of carrying a waterproof backpack, which didn't "breathe," and he ended up with a bad case of "nasty, chafed sweaty backpack back" all day.
That being said, if all you need to carry are some note cards with your reservation numbers, phone, etc., forego a bag (and the time you'll spend in bag check before entering a park) altogether! Wear cargo shorts or capris, and put the sundries in your pockets.
