Be prepared to relax! My DH and I are headed that way in 2 weeks! We've been twice before. DH goes to play golf and I just sit on the porch in a rocking chair and read and relax. It is great!
It is a quiet place but there is plenty of activity if you want to take part in the activities offered. We've not been in September before, only October. It will be warm enough to swim in the pool or head over to the beach. The beach is a short shuttle ride from the resort. Although we are not beach people. Search around on this site and you'll find info on the beach and activities. Someone did a review of a trip they took recently with some really great photos.
There is not a full service restaurant on site, but Hilton Head has plenty to choose from. We have enjoyed Redfish, Sticky Fingers Bar-be-Que, Salty Dog Cafe at South Point, and the pizza place that is close to the resort.
I hope you enjoy your stay.