My wife and I go the last week of August every year. Yes it's hot, yes it's humid, but the crowds are very thin and the parks are manageable. If you go early morning, leave mid-day, and return late afternoon it is very easy to do everything you want to.
On a side note, I am re-evaluating going at his time in 2007. Since Florida passed their law prohibiting schools to start before Labor Day (or something to that effect), I am curious how it will impact the crowds and parks.