Hi, Jennifer! I'm not sure how early in September you plan to be in the parks, and really, most of the parks don't have any type of fall or autumnal decorations in place. The Magic Kingdom, however, is host to Mickey's Not so Scary Halloween Party on various dates throughout September and October. This year, the parties begin occurring on September 14th, so by then, you will see many of the decorations in place that transform the Magic Kingdom into the party location. In years past, I've seen wooden carts, loaded down with fall mums, hay bales, and gorgeous pumpkins, and the banners on the lights throughout the park are changed into a more fall themed banner.
I hope that helps with your vacation planning! If you can catch one of the parties while you are there, I highly recommend them. The party is a must do for my kids each year!
Have a great vacation!