If your travel is limited to less than 7 days, plan on seeing 2 adjacent states rather than trying to see too many states in too few days. I have driven across the Big Mac Bridge but never took the tour over to Macinac Island. Nice but probably expensive. The vacation I remember in Michigan involved paddling a canoe down the AuSable River which was fun.
For Ohio, you could see the worlds largest basket building.
It is the Longaberger basket factory in Newark, Ohio.
Check out when the Kentucky Derby is. Louisville is a great destination. I remember ridding a paddlewheeler up the Ohio in July 1976. The library and campus of Notre Dame is a good place to visit. There are many great places to see in Chicago as well. Pick 2 or 3 places instead of trying to see too much in 6-7 days.

Nearby Mackinac are scenic lake front cities of Harbor Springs, Petosky, and Charlevoix SW of the bridge on the Lake Michigan side. Each is scenic and have moderately priced hotels. Traverse City is also a popular destination with Sleeping Bear Dunes and must see attraction. Other popular mid-state to northern Michigan include Frankenmuth famous for German flavor and many Christmas shops. Crystal Lake south of Traverse City is a beautiful setting with many cottages. Houghton Lake, center state from Traverse is Michigans largest inland lake also with many affordable cottages. You can easily do 4 to 7 days in upstate Michigan and be well entertained.

If you decide on Michigan, I would schedule later rather than earlier. It is cooler that far north.

The heart of Amish country is in Millersburg. http://www.visitamishcountry.com/ In lieu of Put-in-Bay, try Kelleys Island - http://www.kelleysisland.com/
http://consumer.discoverohio.com/ http://ohsweb.ohiohistory.org/places/name.shtml

Hocking Hills in south central Ohio is a beautiful place for hiking and relaxing. Mammoth Cave in Kentucky is interesting if you haven't seen that kind of thing before.

In addition to the great Lower Penninsula places which stumpworks73 mentioned, you might spend a few days in Michigan's Upper Penninsula visiting such attractions as the Soo Locks, Tahquamenon Falls, Whitefish Point & the Great Lakes Shipwreck Museum, Pictured Rocks, Fayette (mining ghost town), lighthouses, etc.

Glad to see you are including Mackinac Island in your plans. Your thread says "We seek any recommendations on scenic areas that have fun places to walk around, short hikes and historical sites to see that could keep us busy 4 to 7 nights". Mackinac Island is mostly state park land. The Island is chalk full of marked and scenic trails. Sounds like Fort Mackinac will be of particular interest to you as well. You will find many lodging and dining options to fit most budgets. Enjoy the ferry ride to and from Mackinac. Your total experience will be quite memorable.

Mackinaw Island is wonderful to visit for a day or two you can also visit Tranverse City and Old Mission Peninsula! In Ohio I love Put in Bay for a day you can take a ferry from there to Kelly's, Millers or Jet Express services the Islands. You ca even visit Sandusky and go to Cedar Point for a day a great park! Hocking Hills in southern Ohio is great for hot tubs and hiking. For the Amish way of life you would want Holmes County in Ohio.

If you visit Amish Country, I would use the Berlin area as a base as it is right in the heart of it all. It is a short distance to Sugarcreek, Walnut Creek (be sure to eat at Der Dutchman Restaurant), Millersburg, Charm and Winesburg.