We did September in 2103, and July last year. Going in September this year, too. Here's our game plan:
Eat a good breakfast in the room and drink water. Bring snacks like Jerky and fresh fruit/vegetables to the park along with 2 bottles of water per person.
Get to the parks around park opening time (9-ish)
Stay until 12:30, head back to the room for lunch and to cool off.
Schedule another fastpass attraction for after 6pm on
MDE app, have lunch. More water.
Swim, do laundry, shower, nap, rest, whatever.... more water. Most likely it will rain at some point between 3pm and 5pm anyway.
Eat dinner around 5... did I mention water?
Head back to the parks around 6pm. Bring another bottle of water per person.
At the parks, have a wide-brimmed hat, lightweight and light colored clothing, sunscreen (and reapply once or twice), sunglasses, and an umbrella in case of rain or needing more shade. Also bring a ziplock baggie of ice and a washcloth per person to cool off with. It feels so nice on the back of the neck, behind the knees, inside of wrists and elbows. I also like to bring Peppermint Essential oil along- it's very cooling.
Walk through the shops or in the shade to get from one attraction to the other, or to get down Main Street.
Drink water every 15-20 minutes or so. I couldn't believe it- last July I went through 4 whole bottles of water per day, PLUS cups of water at every meal. It was unreal, but I needed it!
Take your time and listen to your body. If you feel wonky, take a break in the shade or inside. My favorite out of the way places are:
Magic Kingdom- Train Station (upstairs both inside and outside), Pirates of the Caribbean (just outside the gift shop exit, to the right by the cast member door), Hall of Presidents (inside or outside the exit on the white rocking chairs).
EPCOT- The Land (either inside or outside the doors sitting on the ledge), Morocco (back in the pavilion on the left side where there's the water fountain), China pre-show area (we have been known to sit there and eat a packed lunch for extended periods of time)
Hollywood Studios- Tower of Terror (just outside the exit of the gift shop on the little shaded porch area), One Man's Dream (is that still even open?? if so, great chilly option)
Animal Kingdom- Shaded paths in the Oasis, Small hut to the right as you enter Asia near the water that overlooks Everest in the distance, shaded paths near Tree of Life/Tough to Be a Bug
We have also been known to seek refuge in some nearby hotel lobbies. I love the convention floor of the Contemporary (quiet bathrooms in this area!), and the 2nd/3rd floor sitting areas of the Wilderness Lodge.
For the car, bring a folding sun shade or of nothing else, a towel to throw over the steering wheel.
Have I overthought all this?? Yes, but I have tools to help me cool off when I need them- hopefully this helps someone!