Absolutely do NOT miss Mackinac Island!! Best town I can think of to wander around on foot. Several good CGs nearby; we've stayed at the St. Ignace KOA, which was fine; Straits SP (on the UP near St. Ignace) and Wilderness SP (a bit farther afield, west of Mackinaw City) are nice; several other private CGs in Mackinaw City. Ferries go from both towns for a great day trip to the island. You really do step back in time.
If you have time, the Keweenau Peninsula is a great place to visit Lake Superior. If not, you could consider a visit to Whitefish Point on the eastern end of the UP to count - and it's a fascinating little museum there, dedicated to the various shipwrecks in the Lakes. Camp at Tahquamenon SP (and hike to the waterfall in the park) for a base camp for Whitefish Point.
To visit the Soo, I'd recommend camping at Aune-Osborn County Park, on one end of Sault Ste Marie. Not much to look at as a CG, but the view of the lake freighters going by is stupendous (clean facilities too).