Woodland Park at Grand Marais, MI is pleasant, right on Lake Superior with a nice view of the lake, and has large sites.
The boat tour from Munising along the Pictured Rocks Nat'l Lakeshore is worthwhile, if you have a sunny day for it. If not, try the Apostle Islands, which from photos looks to be quite similar.
I would highly recommend a drive north of Duluth along the lakeshore to Grand Marais MN. There's a big RV park close to shore there, with large sites. Not positive about the state parks along that drive, but the Woodall's online directory says there are large enough campsites at Temperance River, Cascade, and Judge C R Magney State Parks. Make reservations early for any of these. The number of nice waterfalls and high overlooks along this stretch is noteworthy, and there's a nice lighthouse also. In Grand Marais, stroll to Artist's Point (take your camera) and also have a great pizza at Sven and Ole's.
