Go lightweight with comfy straps, and ONE main pouch (to unzip at bag check). Visible organizing pockets inside the bag are okay. I try to eliminate the bag entirely if possible (especially if we return after dinner) After dinner, I used to even skip the camera. Now I just use my phone all the time.
Before we go, I take everything out of my wallet that we won't need on the trip. Just: ID, cc, WDW tickets (if any nowadays), discount cards useful for WDW, insurance cards, and that's about it. Also, anything you can compact, compact - like a minimum of tissues in a snack bag vs. a full box. I would much rather replenish at night what I use, than carry extra all day.
I keep a paper copy of conf #'s in our luggage, The copy I keep with me is electronic. I consult it each A.M.
Water bottle
We only bring snacks we know we'll eat inside of an hour. I don't want to eat food I've been carrying all day...not when WDW has so many fun options. Unless you want it for a medical condition
Rain poncho -only rainy days
Sunscreen - a 1/2 ounce chapstick style tube. Really only use if someone forget to apply in the A.M. , and maybe on Epcot days.
Hand sanitizer - Don't need, but if I have a 1/2 ounce sample size in my tiny purse. Washing hands is better.
Band-Aids, moleskin, antibiotic salve, & Ibuprofen or naproxen - If have a bag, I carry two of each. wrapped in micro plastic bag. You are at WDW, every park has a fist aid station with free wound care, etc. Many stores carry small packs of meds for like $2.
Medications & supplies as needed for chronic medical conditions - YES! - if you need. I have read horror stories on the DIS!
At times, we have also used zip lock bags in the parks: AK days, or days when it will rain hard. I ALWAYS pack ziplocs in our luggage. For travel, the hard to find, 2.5 gallon bags are great, as are the tiny snack sized bags.
Zippered pockets on clothes are a godsend. Cargo shorts for women are kind of out of fashion now. They were great at WDW. If it is cold or wet, I often toss all of the above in my coat pockets and love it.
Also, having learned the hard way, I avoid anything that can come apart, many bags have a strap that can be removed. You want sturdy straps.
It also matter slightly when you are going. If spring, you can really carry a minimum. If summer, bring extra water.
Water tolerant shoes are also smart on wet days.
If I must have a bag, I like cross body. I don't have to do anything on most rides (except at US). US is very strict. They have free lockers.
