September seems to my family's time to go as well. It is hot and humid- so pace yourself. At night though it may cool off- down around 70!! Make sure to drinks lots of liquids, especially water. We go back to our room during the middle part of the day to take a nap or just relax.
We have been to WDW several times in September and it is HOT and HUMID, but we think less rain that summer visits and usually less people.
Bring your swimsuits, shorts and lots of sunscreen!