When considering the time of the year to go I would suggest that you go after July 4th. I used to live in Marquette and the early part of the summer has problems with swarms of black flies and no-seeums. Mosquitos are always there so you can't avoid them until there is a good frost.
That being said, I suggest a day visit to Mackinac Island. Camping can be a bit difficult around the 4th of July in that area, but there are several good CGs above and below the bridge. The Soo Locks in Sault Ste Marie is a good visit too. The Munising area has the Pictured Rocks National Lakeshore & be sure to take the cruise. There are also several nice waterfalls in the Munising area. If you continue west along M-28 you'll go through Marquette. There isn't much to see in Marquette, although the upper harbor area where they load the large ore boats is interesting and Presque Isle Park is right next to the dock.
The Keweenaw Peninsula (that jut of land that goes up into Lake Superior) is an old mining area where you can visit a mine. Going all the way to the top, Copper Harbor, is a long drive, but there are some nice campgrounds in the area as well as Fort Wilkins State Park. In the Ontonagon area, at the west end of the U.P., is the Porcupine Mountains State Park. If you like hiking this park has some excellent trails. I prefer hiking both sides of the Presque Isle River at the south end of the park. For camping in this area we like Ontonagon Township Park on the north side of Ontonagon. The state park is ok, but not preferred CG.
Below is a nice FHU campground west of Marquette about 40 miles. It's on a very large lake in a very rural area.
http://www.michigammeshores.com/For groceries be sure to stock up when and where possible. Marquette has good shopping as does Sault Ste Marie, Escanaba, the Houghton/Hancock area, and Iron Mountain. The same goes for fuel, although they do have diesel available, but the prices can be high, especially in the northern half of the U.P. Usually found good prices in Manistique and Escanaba.
Have a nice trip!