Will you have time to visit Mackinac Island while in Mackinaw City? Have a Pastie in MC; a local specialty.
Recommend staying at the Municipal campground in Medora because it has a pool. Take the Ranger tour that discusses why the Prairie Dogs and Bison get along so well together. Drive around a look for the wild horses.
In Glacier hope you are up to doing some hiking. Much better to get to know the area than just driving along the GTTSR. We were there one August and someone had built a small snow man along the trail. Good boots and hiking sticks are a plus.