You stated 'upper Michigan'. I would suggest 'northern' Michigan in 'lower Michigan'. Confused?? :)
All along the western part of the state is a gorgeous Lake Michigan lakeshore. Our favorite area in around Traverse City, including Sleeping Bear Dunes National Lakeshore (early reservation a must) and the Petosky area. You won't believe the beauty up there. Small quaint towns with fun festivals going on all summer. That whole area is very popular so reservations should be made ASAP.
If you have additional time, which I doubt, then cross over the Mackinac Bridge to 'upper' Michigan. You wouldn't have time to do justice to the whole Upper Peninsula on your timeframe. Stop and enjoy the beauty! If you want to do a little of the UP then head to Taquamanon Falls State Park and from there drive up to Whitefish Point and tour the Edmund Fitzgerald Museum. Then it'll be time to head home.
This would definitely be a 2-week trip.
Have fun planning!