Mackinaw City in September is beautiful. If you're driving yourself up or at least will have access to a vehicle, make sure to take the Tunnel of Trees from the little town of Harbor Springs up to Cross Village.
The trees grow like a tunnel and if you're lucky, you might hit it right when they're all in full color. It's very magical.
Harbor Springs is one of those towns where all the money from Detroit used to spend their summers. The homes look like small hotels and I've always been amazed that many of them close up in late summer only having been used once.
Cross Village has this funky little restaurant called the Legs Inn. It's a must because of the way the guy built it.
Mackinaw City, and Mackinaw Island are both beautiful. You could also make a day trip up to Tahquamenon Falls
It's probably 50 miles from Mackinaw, but well worth the drive. Make sure to plan enough time to see both upper and lower falls.
I could give you places to go photograph all day long in that area, as this is where Cindy and I spend many weekends. It's a few (4) hours from our house in MI.
Hope you have fun!