We have gone in early September a few times after Labor Day and you will have some of the lowest crowd levels of the year! You will still have the heat and humidity of the summer but since the lines will be so short in the mornings you should be able to get plenty done; then enjoy your resort pool in the afternoon. You will be refreshed to then go back out in the evenings!
Tip: Dont make your FastPass Pluses for early morning since most rides will just be a walk on. Set them up for early afternoon if you are not taking a break at your resort or evening if you are.
Read some of the early September trip reports to get a good idea what to expect.