The L. Superior shoreline in MN, from Duluth to the Canada line, is especially nice. Many waterfalls, good views of the lake, a high overlook at the Palisades, and Artist's Point at Grand Marais. Granted, this may be a bit of a detour for you, but it's a destination all its own.
In Michigan, the Porcupine Mountains SP (they're hills, really) is nice, as is McLain SP (right on the water). The drive along the north tip of the Keweenaw Peninsula, Eagle River to Copper Harbor, is scenic. You'll see the lake around Munising, and the Pictured Rocks boat tour is (expensive but) worthwhile. If you then move south to US-2, try to stop at Palms Book SP and see Kitchi-Ti-Kipi, the Big Spring. You'll see some more lakeshore around Manistique and again as you approach St. Ignace. Then prepare to be amazed by the Mackinaw Bridge crossing.