The one nice thing about the summer is the daily rain shower: sure, you can get soaked and things may be shut down for a while but it usually clears out a crowd and is done in an hour: giving a break in the humidity and cooling everything off in the evening. Basically like everyone said here: you're up and active in the morning when it's "cool" (I'm from Texas, 75 is "cool"), then find lunch + long a/c filled rides + shopping as soon as it starts getting unbearable or head back to the hotel if it isn't a million miles away, and then back out after that 4:00(ish) storm, when it's cooled off again. Free ice-water. Lots of ice-water. Repeat viewings of Impressions De France. Cutting through shops to steal A/C, etc. Also just: listening to your body. I think it comes from growing up in the heat of the south, but I had to train my Swedish wife how to recognize the signs of heat exhaustion because no one had ever taught her about them. Know the signs and address them EARLY. If you're too hot - change plans and do something else that is in the A/C.
I grew up going in the summer though, and while I'm sure it was hot - those are definitely not the memories I have! Taking my first summer trip as an adult this year and the Swedish wife is panicking a little about the heat, so we're just planning to take it nice and slow and have the freedom to bail if needed. It's Disney -- a hot, humid day there is still a heck of a lot better than a hot, humid day back home!