Glad you posted this topic. This question and personal bag issue has plagued me for years. Every year I've tried something different, and then this year it hit me. I no longer take a dedicated camera bag with me. One, I like to carry a narrower easy to stow bag with me that has everything we need. It has to be large enough to fit my 5D in an emergency, like a huge downpour, or if I'm just tired of shooting. I stopped taking more than one lens with me to the park also. Now-a-days I only take the 24-105mm, and a 50mm f1.2 prime. The prime stays in the hotel most of the trip, but I'll take it for a day here and there just for nice portrait shots. 95% of the time I keep the walkaround lens on it.
OK, back to the bag. I'm retired Army, so my day pack has to be durable, easy to stow, and have a ton of pockets. I recently went with a CamelBak Hawg. See picture below:
It's perfect for Disney. It carries 3 liters of water, if you want to, and the hydration cap is so large you ca stuff it full of ice first. It's big enough for all your day needs, extra camera stuff, and personal items like ponchos and an umbrella. It's also small and easy to take on any ride without it getting in the way. Took it on Expedition Everest, Space Mountain, and all the other rides I can think of without any problems. Camera went inside during the roller-coasters and back on my person after using a BlackRapid's Sport strap.
For days you don't want to take a bag, but fear that your camera could get wet when it rains, or if you go on a ride like Splash Mountain, try one of these. See picture below:
These are called Dry Bags. They're awesome! When not in use they roll up and clip together to keep it from unrolling. When I didn't take my backpack I put one of these in my pocket. On a few occasions I would unroll one of these, a SMALL (fits a Canon 5D with 24-105mm easily) then around the strap wrap the top of the clip part, and clip it around the strap making it snug to keep rain out. No worries about my gear getting wet, and I didn't have to carry a bag all day. You can order these on eBay if you like them. As far as a "camera bag" goes, I never use one. I want an all purpose pack for the park, and not just a bag that's sole mission is camera equipment.