Having just been up in your neck of the woods, I'd suggest the whole Mackinaw area at the tip of Michigan. There are also a couple of nice islands up there... I forget the names... possibly Washington
and Beaver Islands, I think. The important part, like everyone else says, is not to "get somewhere", but to take your time and enjoy your month off.
Also, watch water
depths in those lakes. We had a lot of trouble with marinas
not being accurate about their depth
(mostly at fuel
docks). Also, get a crusing guide. Everyone up there kept telling us to use Richardson's. Yet... nobody stocked it or knew where to find it. ha ha ha