As someone else has said, you should definitley take advantage of the extra magic hours specifically for resort guests. This way you can probably ride twice as many rides compared to if you didn't use the extra magic hours...it's a huge advantage. September is usually a relatively empty time of the year at WDW anyway, so you should be able to ride and see everything you plan to. The great thing about Disney parks is that, even if you don't ride many attractions, there is so much else to see and do. Make sure you check out these rides if you can:
Space Mountain (Magic Kingdom)
Thunder Mountain (Magic Kingdom)
Splash Mountain (Magic Kingdom)
Rock 'N' Rollercoaster (MGM)
Tower of Terror (MGM)
Test Track (Epcot)
Disnosaur! (Animal Kingdom)
Expedition Everest (Animal Kingom)
If there is ever a day when you are at Magic Kingom while it is very busy, be sure to ride the Tomorrowland Transit Authority (which is a blue monorail-typed ride which towers over Tomorrowland and never usually has more than a 10 minute queue, even on the busiest of days). This probably has to be the most underrated ride in the Magic Kingdom...it gives you a tour of almost everything in Tomorrowland, including a great view INSIDE Space Mountain.
Here are some new rides at WDW which, IMO, are quite a waste of time(except if you have got a lot of time to spend). (Biggest waste of time first, least waste of time last):
Stitch! (Same as the ride it replaced, just new theming)
Mission: Space
Soarin'
I'm not suggesting that these rides are terrible, but they are just not as good as I would have expected.
Typhoon Lagoon and Blizzard Beach are also great water parks which shouldn't be missed.
You'll have a fantastic time! There's no doubt about that!