Mackinac Island is an island in Lake Huron, east of the Mackinaw Bridge. It has a lot of history, most notably the War of 1812 as it was controlled by both Americans and British. There is a historic Fort and cemetery there. It is fairly small, there is a trail for walking or biking all the way around that is about 7 miles. there is some beautiful natural history, ie Arch Rock as well.
The movie Somewhere in Time with Christopher Reeve and Jane Seymour was filmed at the Grand Hotel. It is very large, very expensive and very formal. There are other places to stay on the island though, hotels, Bed and Breakfasts, etc.
The downtown area has some great pubs and a lot of souvinier type shops, especially Fudge- thats kind of the signature thing - Mackinaw island Fudge.
Transportation on the island is limited to walking. biking or horse carriage. You can rent bikes cheaply at most of the hotels.
Are you used to cooler weather Michigan style? October, depending, might be too cold for you. The ferries stop running about a week before Halloween. You can take an airplane in the winter, or a snowmobile when the lake freezes. I have not been there in winter, but I hear it is pretty nice because only a few places are open, a few people are there and you see the same people at dinners etc. and get to know them.
The ferries leave from St.Ignace in the UP or Mackinac City in the LP. Both of which you get to by taking I-75 North. Mackinac City has a lot of nice cabins and Band B's and also some more history - Forts, Lighthouse.
You should also head into the UP and see Tahquemenon Falls and whitefish point. They are not too far from the bridge. The Falls are gorgeous and there is a nice brewery at the lower falls. Whitefish Point has a Shipwreck museum.
I would definitely recommend going up there, I have been many times, I was there in October this past year and it was in peak fall color. Amazing. I have a lot of personal photos of the area if you would like to see them just email me.
Make sure to have some Fudge, some Whitefish and some Pasties while you are there. Oh and some Cherry Wine. Gotta have the full taste of Michigan.
:)