The nicest sandy beach stretches of big-lake shoreline tend to be on the western shores of Michigan. This is the Lake Michigan shore from Benton Harbor to Petoskey. There are also some nice views and sandy beach from St. Ignace to Engadine. If this is something you like, you may want to drive the extra miles and pick up US-31 northbound. If not, well, all of Michigan is beautiful.
I'm not sure how far north and west you want to go in the Upper Peninsula (the UP), but we enjoyed staying at the Ontonagon municipal CG. It's right on Lake Superior, the beach is nice, and the place was clean and pleasant when we were there. Porcupine Mountains are nearby, and Lake of the Clouds is picturesque. Also, north of Houghton is McLain SP, which is also on the lake and very pleasant (and the campsites are sought after!); the shoreline drive on M-26 to the tip of the Keeweenaw Peninsula is rocky rather than sandy, but beautiful.