I did such a trip in the second half of June. The top part of Michigan Upper Peninsula (roads 31 and 119 on the west side) is worthwhile to visit and the fall colors should add to the scenery.
When you cross the border to Sault Ste Marie, from there it is about 2-3 hours to the Lake Superior Park which offers several hikes along fast rivers with waterfalls and also some pretty, but demanding coastal trails. If you have time, you can drive to Wawa which is just north of the Lake Superior Provincial Park. The scenery and colours on TransCanada Highway (did you notice, I switched to Canadian spelling north of the border?) are spectacular by end of September. If you don't want to drive that far, there is a Chipewa river with a respectable set of waterfalls about one hour north of Sault Ste Marie on Hwy 17.
The other possibility is to drive south towards Sudbury and then to Manitoulin island which has a few small waterfalls and several lighthouses. Or if you like hiking and paddling, Killarney Provincial Park with its many lakes and hiking trails is just one hour southwest from Sudbury and should be spectacular, too. If you take this route, you can return to USA via Sarnia rather than backtracking through the Michigan Upper Peninsula.
