I bring the same stuff to the parks if I am on-site or off-site. If off-site - extra "might needs" are in the car, while when on-site, those things might be at my resort or put in a locker.
I've used ALL types of methods of carry - cargo shorts, fanny bag, cross-body bag, lightweight string backpack, school book size backpack. And I bring all as options to carry each trip depending on the plan of the day and the weather forecast. VERY key is the weight of the bag, too. I think the
Loungefly backpacks, for example, are WAY too heavy of bag - especially for the amount of stuff they can hold. They are sure cute - but ridiculous heavy, IMO.
Here's how I think about this:
1) Arrival day - likely my cross-body bag because that is my normal purse that I need for flying to WDW that day. I typically drop my backpack at the resort which likely held my cross-body bag during my flight.
2) Weather is going to range from 60 degrees to 80 degrees to 50 degrees in one day - full size backpack that I most like put in a locker (except for EPCOT) so that I can change clothes for the appropriate temp during the day.
3) Hot day, no threat of rain - cargo shorts or fanny pack depending on how long of a day I am planning.
4) A day I expect to buy a small souvenir and I don't want to/can't wait for package pick-up or resort delivery - string backpack.
5) Headed to DS for some reason and a park that day - cross-body bag or string backpack.
6) I'm exhausted day and my back hurts - fanny pack or cargo shorts because my other bags are heavier.
7) Hot day, I'm sick of "the FL water taste" or "paying for water" day, string backpack so I can start my day with three bottles of water!
It's about the weather, what I need/want to carry that day, and well, my mood.