Is there a cruise port that is close to you that would be more convenient to cruise from, or are you willing to fly anywhere to board the ship? Knowing where you are leaving from will focus your choices to the ships that sail from that port.
Aside from the obvious eastern or western Caribbean itineraries, Bermuda is still an option in September, but it only sails from NYC or Boston I believe. It is a great choice as you are in port coming and going at will for nearly 3 full days. One way cruises from the east coast into Canada are also an option in the fall. We also love the southern Caribbean itineraries.
Do opt for a full 7 days if you can. Ask your TA lots of questions, and think about what is important to you on this cruise. Do you like lots of entertainment options available on the ship? Cruiselines vary somewhat in their atmosphere and passenger demographic. Sme may be very sedate after 9:00 pm. Would you plan on taking lots of very active excursions while in port, or do you just prefer beautiful beaches?