Hi Kristin,
September's weather is wonderful with highs in the upper 80's and lows in the 70's. I always pack a rain jacket for everyone as you can have afternoon showers. The crowds are not too bad as most children are back in school. I usually consult the
Park Hours Calendar and schedule when I am going to each theme park based on Extra Magic Hours. For example, I would go to Magic Kingdom on September 16th as the park is open to Midnight. I also plan my dining based on what park I will be going to that day.
Mickey's Not-So-Scary Halloween Party usually will be starting around this time, and is one of my family's favorite events to attend. You are able to Trick-or-Treat in the Magic Kingdom and can dress up as some of the characters are in costume as well. This requires a separate ticket.
For a park schedule, I would suggest going to Epcot on September 15 and 19, Magic Kingdom on September 16 and 18, Animal Kingdom on September 17 and 21, and Hollywood Studios on September 20th based on Extra Magic Hours. Downtown Disney is also such a fun place to visit and shop, and you could always spend a half day there after Hollywood Studios.
I have so much more to say but have run out of room. Please send me a follow up question, Kristin, if I can help you further. I hope you have a magical vacation!