Nantucket in September can feel like a summer day, but tends to cool down at night. She is called the grey lady for a reason as it is frequently foggy. Pack options.
Nantucket is 15 miles long from east to west so it’s usually pretty easy to get anywhere. There are Ubers, taxis, and bike paths with lots of bike rental options. You can bring your car over on the slow ferry, but reservations need to be made well in advance. There are also rental cars. That being said you shouldn’t need a car for the weekend.