So . . . yeah . . . the lure of WDW is too much to resist. We are indeed going. We decided to stay at AKL, so if we end up at the resort/in the room most of the day, it'll be fantastic.
Many many MANY thanks to everyone who posted!
We have many summer trips under our belt. Every year this forum has threads like this. To me, WDW in summer feels like summer. There have been a few trips where the humidity/temps were especially high, but overall we find WDW tolerable. The biggest downside is the afternoon storms are common.
One tip is to acclimate yourself before you go to 80 or higher. Get outdoors. Limit use of air conditioning. Try to get used to 75 or 80 as an indoor temp, then 90won't feel so different. People have lived in warm climates for thousands of years without AC. Acclimation helps.
We do tend to take afternoon hotel breaks in summer, but a big part of that is to dodge some of the storms. If you are only going to parks starting at 4pm, then storms might = missing out. Many attractions close during lightning, and all the boats and pools close.
I advise good rain gear and water-tolerant walking shoes. Avoid too much denim and sneakers on days it is supposed to rain. (or swap to sandals for the PM.) Do watch the weather.
We also love what are called packable rain jackets. They fold down to the size of a pocket. Otherwise, we don't really go out of our way to buy special clothing, except footwear. Mister fans = clammy, IMO.
On days it is likely to rain, we bring the jackets. On days it is not likely to rain, we carry plastic ponchos. But this we do all times of year at WDW. Often summer rain is intense, but short lived, so waiting it out makes sense.
For summer trips, we get an early start. We eat lunch about 1:30-2pm. After lunch = a bit sleepy, so that's part of afternoon breaks as well. Siesta, swimming, re-hydrating. Then we return about 6pm.
Do drink plenty of water, but also make sure you are getting enough electrolytes. Eat plenty of fresh fruit! Drink apple juice, lemonade, other fruit juices. Though the WDW lemonade is often the kind with fake sweetener, so it is really smart if you have your own supply of good beverages! Limit soda, coffee, fake sweeteners, sugar-bombs, and alcohol! Do splurge on the expensive Voss water they sell in Epcot's Norway! Try to avoid/limit drink packets, unless they are the electrolyte hydration kind w/ no fake sugar. Pineapple juice is also great, where you can find it at WDW.
Along the same lines, I think it really pays to limit junk food. that's a tall order at WDW, but produce = hydration. Summer = lots of great summer produce. We try to eat lots of fresh fruit and veggies all summer, including WDW, and I think that probably helps.
Last, it pays to take photos early in the day, before faces look a bit sweaty, and/or you maybe look a bit ragged after getting caught in the rain. A change of clothes can be helpful if you plan nicer TS dinners.