I have a sciatic nerve problem and bad knees so good shoes are essential for me. I hate
crocs as i have found them to give me blisters in the long run.
I am a flip flop lover too, so for WDW, I ordered "Reef's Girl Slaps" on amazon.com and got them delivered directly to my resort. Reefs are high quality surf sandals and have a thick cushioned sole with arch and thick strap for comfort and stability. They cost me half the price then if I would have got them in store. they provide amazing foot support. Best of all they are waterproof so if you are caught in a freak thunder shower, they dry immediately. I have worn them on three trips to WDW and my feet stay cool and dry and no aching feet, back or knees.
Another shoe I recommend is "Sketchers: Go Walk". They are made to be walked on for long periods of time and are the only shoe recommended for people with sciatic nerve problems. They come in a runner or dressy shoe or Mary Jane style. I own both the runner and the Mary Jane. They are amazingly comfortable, lightweight and breathable. They have a cushioned base with a inverted curve so that it follows the foots natural walking motion. I wore them on my last trip in December as it was cooler weather and not appropriate for my flip flops. Not one day of sciatic pain or sore feet. I have converted all my coworkers at work since we are on our feet constantly. Now we are a group of 10 wearing them everyday
I used to wear Keens before and they were great for comfort and waterproofing however my feet used to get really hot and sweaty and the same could be said for regular runners.
I hope this helps. Have a great trip!