Our first WDW trip was a late August trip. If you can go towards the end of the month, the crowd levels definitely start to taper off. I won't downplay the heat and humidity though. Hydrate, hydrate, hydrate; and take a/c breaks often or do more indoor, air conditioned attractions/shows if you are in the parks in the afternoon. I remember it already being in the mid 80's when we would head out to a park 8:00 in the morning and it feels like the heat and humidity hit you when you walk out the door, lol! It does take some getting accustomed to and I would plan our park days to get an early morning start and leave in the afternoon to get a break from the heat and nap and/or enjoy some pool time, (especially before the daily rain/downpour hits) then return to the park in the evenings when it is a little more comfortable. Another idea is to start your park touring in the early evenings and stay until the parks close (if your travel party can handle late nights). The longer park hours definitely make these plans doable. Our family's next WDW trip in a couple years will most likely be another August trip. It's definitely manageable, you just have to be prepared and have the right expectations. Have fun!