One of my favorite spots in the Upper Peninsula is Au Train Lake, halfway between Marquette and Munising. It's lightly developed, so you can enjoy the amenities of a rented cabin and a nearby convenience store amid the depths of the Northwoods.
There's also a National Forest campground at the undeveloped south end of the lake. At the north end, the winding Au Train river flows out toward Lake Superior, providing some excellent canoeing (rentals available). At the nearby mouth of the river, there's a beautiful white-sand beach on the big lake where the river outflow moderates water temperatures -- one of those rare spots on Superior where swimming is comfortable.
For a change of pace, Marquette is nearby, a bustling little college town. In the other direction, scenic Munising lies at one end of Pictured Rocks National Lakeshore. Tour boats run along the cliffs hourly in the high season.