Detroit area: the Henry Ford Museum is very good. Consider camping for a night (or more) at Algonac SP where you can see the ships moving up and down the St. Clair River.
West side: plenty of sandy beaches. Silver Lake SP has dunes that you can drive on (rent a dune buggy or use personal 4WD), and 2 miles south of the dunes is a nice lighthouse, Little Sable Light. You'll find even more sand at Sleeping Bear Dunes, where you can drive to the top in one place (the Pierce Stocking Scenic Drive) or just climb the dune; there's nice swimming spot just north of Empire at North Bar Lake, it's a small lake so the water is warm in summer but you can walk about 100 yards to the L. Michigan shore for a colder dip.
There are some great rivers for canoeing or kayaking, or just plain floating downstream. Some others will probably name the ones on the west side. I'm more familiar with the Rifle River on the east, which winds up near Standish (my old stompin' ground). Definitely a good way to spend a summer day.
Farther north on the east side, near Oscoda, is the AuSable River. Well respected trout fishing waters. The Lumbermen's Monument is worth a stop. While there you can get a campsite also, if you like. I recommend this one, for which you must go west, then north, then back east (it's on the north side of the river), then take a sandy 2-track for a few hundred yards to reach this site...
There's a nice state park at Harrisville. Then if you want to visit a fairly secluded and quiet L. Huron beach, check out Negwegon SP. Last I knew, you had to drive another sandy 2-track to reach this place, so I rarely saw more than 3 or 4 other people there, and sometimes none.
A bit further north, the Presque Isle Lights are worth seeing. The old lighthouse is no longer used, but it's quite picturesque, and you can climb up inside the new lighthouse just up the road.
West of Rogers City, Ocqueoc Falls is pretty even though small, and some folks swim there.
Mackinaw City is touristy, with many small shops you can walk around at. Smell the fudge being made, and caramel corn. Buy souvenirs. At the east end, before going in to the dock parking, walk north a block to the local bakery and pick up some of the best homestyle cinnamon bread ever, or some pecan rolls... yum! There's a park along the north shoreline with another lighthouse, and a great view of the Mackinaw Bridge. There's a fort in town, and another fort on Mackinac Island... take a ferry to this no-motor-vehicles island for a trip back in time; I recommend a bicycle ride around the island. Many people have recommended camping at Mill Creek RV Resort, or you can go west of town to Wilderness SP.
A nice drive will take you into Petoskey where there are many large old homes built by the well-to-do decades ago. And drive the Tunnel of Trees north of Harbor Springs.
In da U.P. (yah, dat's how dey say it), there's tons more to see and do. Along L. Michigan you have a nice open beach along US 2 at one point, stop and feed the gulls and cool your feet in the water. The Seney Wildlife Refuge is north of Engadine. More state parks near Manistique if you are looking to camp, and be sure to day-visit Palms Book SP to see the Big Spring... 40 feet down in water so clear, you can see the fish at the bottom.
Continuing west and then a bit south, see the historic buildings and artifacts at Fayette SP.
Closer to the L. Superior side, you have the Soo Locks at Sault Ste. Marie, watch the big ships working their way through the locks. If your rig isn't too big, you can ferry across to Neebish Island and camp on a quiet island. Pleasant camping at Brimley SP also. Then at Paradise you have Tahquamenon Falls, and also visit the lighthouse-turned-museum at Whitefish Point. Woodland Park CG at Grand Marais affords a good view of the lake. At Munising, the boat ride along Pictured Rocks is entertaining; ideally, try to take the sunset cruise on a calm, sunny day. Bay Furnace CG west of town is supposed to be nice (I haven't been there yet, it's on my list).
Continuing west, the Keweenaw Peninsula is worth a visit. McLain SP is along the water. Drive M-26 along the shore for some great lake views, and there's a waterfall just off the road plus an interesting looking monastery. Buy some thimbleberry jam (it's similar to raspberry). Ft. Wilkins is another good place to visit and/or camp. Then you have the Porcupine Mountains; the east unit features Lake of the Clouds, and the west unit has a great hike along a river with 3 small waterfalls in succession.
Oh, and be sure to visit Bond Falls near Paulding, it's my favorite waterfall in Michigan even if not so famous as Tahquamenon.
I'm sure I have missed more than half of the good stuff!
Mike G.
Liberty is meaningless where the right to utter one's thoughts and opinions has ceased to exist. That, of all rights, is the dread of tyrants. --Frederick Douglass
photo: Yosemite Valley view from Taft Point