My suggestions (just got back yesterday):
Read through all of these suggestions before this one and decide which ones work best for you.
Packs and Backpacks:
Everybody in my family (4 of us) had packs that we all carried something we wanted right on ourselves. Mine had my wallet and phone, plus a ziplock back to place the pack in on the wet rides. My wife and kids had their own for misc. things.
I carried the backpack the majority of the time. That had the ponchos for everybody, a couple of sweatshirts (if you do get wet and go into a cool building, you might need a sweatshirt, my wife gets cold easily), a towel, sunscreen. Plus in the backpack I would store the camera and mouse ears on the wet rides. On a couple of days we checked the backpack.
Are packs "fashion faux paus" or whatever? Maybe, but I am at a theme park to have fun. It isn't a fashion show and I spent my 10 days wearing mouse ears on my head...if you want to make fun of a pack you can't really see under my shirt, then you have a bigger problem than I do.
Sandles, tennis shoes, flip flops???
I wore sandles the majority of the time. They are the kind that are NOT exposing my toes. But it was dang near 100 degrees (if not more than that) and way too hot with normal shoes and socks on. Plus, there are few things in life more uncomfortable than wet tennis shoes and socks. I almost bought Crocs...but the price is sooooo high. But they seem like the ideal shoe for the theme park thing. From what I am told, they are really comfortable, they don't get wet, and they have ventilation. The only problem is that they are ugly as sin. But as for fashion, see my earlier entry.
LISTEN TO WHAT EVERYBODY SAYS ABOUT HEADING OUT AND RELAXING!!!
I didn't listen. I did fine. I also run 6 miles a day though. My wife and kids were beaten to heck with my continual "go, go, go" ways. They went along, bless 'em, but yesterday my son fell asleep at 1:30PM on our way home (after the plane flight) and didn't wake up until this morning at 7AM. The poor little guy was a beast on many of the days, and as it turns out it was all due to his just being exhausted.
Oh yeah, and adults, DON'T BE AFRAID TO BE HAVING FUN! I am amazed at how many adults I see there that are "too grown up" or whatever to just give in and be a kid again. Get some ears, put 'em on, and hug a character.