Savannah is a “big” small town. You will have no trouble getting it figured out quickly.
Downtown is made up of squares. Research that and figure out if there’s anything you’d want to see there, many have some type of history to them. There’s plenty of cafes and restaurants. Carriage and bus tours. Historical sites such as Bonaventure Cemetery (as mentioned) and homes (such as Johnny Mercer house). Broughton street (shopping and restaurants) and River Street (cobblestone River front with food/bars/restaurants and sometimes street entertainment).
Midtown is mostly homes, some restaurants. Hunter is kinda midtown/southside border.
Southside is a more mall/hospitals/college (GSU), “city” type area...think Best Buy, dicks, grocery stores, etc. not scenic, but restaurants and hotel options.
Then out from downtown is whitemarsh, Wilmington, and Tybee islands.
There are also shows/performances at the Savannah theater that are a good time.
September will be hot and humid but Savannah is always pretty and enjoyable.