I would never wear anything but sneakers in the parks. I usually pack a pair of flip flops just for walking around the resort, but I would never wear them in the parks. Too many things that can go wrong with flip flops. I trip wearing flip flops just walking around at home; it would be a mess with the amount of walking I do in Disney, plus all of the climbing in and out of ride vehicles, a ton of stairs everywhere, etc. They also fall off too easily. If you're walking in the big mass of people leaving the park at the end of the night and your flip flop falls off, it's not going to be very easy to just stop in the middle of the crowd and put it back on without getting trampled. Also, there are a lot of other feet and wheels that can step on/roll over your feet in the parks. I wouldn't go with any open toed shoes. Sneakers will at least provide a little bit of protection.