I was just back in the midwest in NE Indiana in July. Plan was to visit Brown County State Park in the southern part of the state, about a 4.5-5 hour drive from where I was. Unfortunately didn't make it due to an accident that affected my trailer, but still somewhere that I'd like to see.
The Brown County area has a lot of things to see and do. Notably a goodly number of covered bridges. Also a hilly area with what looks to be some great sightseeing, small towns to visit and shop.
I've read about (but never been to) the Hocking Hills area in Ohio, another apparently popular area that's not in Michigan.
There's a lot of Amish areas in Indiana and Ohio, so a trip to camp/visit could be something of interest.
Not to detract from Michigan, because there's a lot to see and do up there, just that there are other areas as well.