I know you are probally talking about Orlando but Im from Ireland and have been going to
Disneyland California every June for the last number of years. I typically spend about 14 hours per day in the parks.
I wear well wore sketchers runners and socks for the parks. You dont realise it but standing and walking on that baking hot concrete really does leave your feet in bits. The concrete absorbs the heat and for those of us from UK and Ireland our feet and legs are just not used to that. Every night after a park day, no matter how late, I always take a bath. You have no idea how good it feels to soak the aching feet. I use a foot file in the bath, which really gives my sore feet a good massage. You might also be unlucky enough to get Disney legs. This is an itchy red rash on calf's caused by walking and standing on the hot concrete. Use an over the counter antihistamine if this happens.
I wear black leggings from Primark with a loose oversized t shirt or spaghetti strap style top. I bring a light shrug to cover my arms and shoulders during the hottest part of the day from 12pm to 4pm.
I have a very small backpack which is just big enough to hold 2 bottles of water, my sunscreen, money etc. I found having a large or medium sized backpack or an over the shoulder bag far too uncomfortable in the heat so over the years I have downsized and downsized and now just have something around this size
I wear Boots Soltan Factor 50 on both my face and body. I reapply about every 2 hours. I find a restroom and splash cold water on my face and wrists to cool down. I have travel size packet of facewipes in my bag and I clean my face and underarms. I reapply my sunscreen and deodorant. You would be amazed at the difference that 5 minute freshen up makes.
I also have a hand held fan in my bag, like this. Some people use a battery fan, but I have found this works really well, especially when you are in a hot queue waiting for a ride.
I plan my park days well. I do as much rides as I can between opening and lunch time. Between 12.30pm and 4.30pm, the hottest part of the day I do indoors stuff, table service meal, shopping, shows or just find a shady or indoors place to sit and read a book or people watch.
I alternate between drinking water and coke/ fanta/ gaterade. Gatorade is kinda like the Lucozade brand, they have different flavours but I usually stick to the Isotonic one to replace the salts and sugars in my body as I sweat so much. Its really important to stay hydrated, our Irish /UK bodies are just not used to be so active in that high heat. And yes I find I pee more often that I would at home.
I used to wear a large sun hat but I found it too much trouble. Im a small person and I would have to take it off to talk to people as my eye level is lower. Then there was the issue of how to carry it when I wasnt wearing it. Now I just wear sunglasses and no hat. I am just very aware of making sure my face is well protected by sunscreen.
Do your shopping at the end of the day. The shops in Disney stay open to park closing and they restock through out the day. In Disney they put your shopping into plastic bags. Carrying even one or two plastic shopping bags in the heat gets uncomfortable real fast. The plastic just makes your hands sweat more and then they bang off your legs and it just makes you irritable. If you have to buy something during the day, rent a locker and store it instead of lugging it around in the heat.