Fort Michilimackinac in Mackinaw City, just after you cross the bridge, is worth a stop if your are interested in the fur trading history of the area. Fort Mackinaw on the island is also good if you are interested in history. Cover your ears when they fire the cannons.
Rent a bike and ride all the way around the island. It's not a tough ride and only takes a few hours with stops along the way. Beautiful waterside views, and if it's good and clear, a nice view of the bridge too.
I know you said you already had all your reservations; very good move; but the dark sky park at Wilderness State Park is something to check out too. Not sure exactly what programs they have going right now.
For touristy shopping Petoskey and Charlevoix will likely be right on your path, as will Traverse City. Petoskey is the larger and has Walmart, etc if you need to restock. You can also restock in Traverse City an hour or so closer to Sleeping Bear. You'll find both Costco and Sam's there if you are inclined, along with lots of other shopping and fun.
The redeveloped Old State Hospital grounds in Traverse City are another unusual stop. They include a botanical garden (free) that is still a work in progress, an arboretum of lovely old trees, unique boutiques and restaurants in what was once part of the state psychiatric hospital facility. The old architecture of the buildings is amazing and beautiful. You can even take tours of the "catacombs" if that type of thing interests you and learn the history of this very unusual place. It's a very interesting study in truly forward thinking reuse of old defunct buildings.
Moomers ice cream (on the west side of Traverse City) is a great local family operation which was voted best ice cream in the US a few years ago by Good Morning America. Of course, all the tourists go, but so do we locals. Too may great flavors to tell you my favorite, but Cherries Moobilee is a hot seller.
Sleeping Bear (just 10 minutes from my home) has soooo much to do. The Pierce Stocking Drive is beautiful. Plan a lunch at one of the picnic areas and be sure to check out the Dune overlook. Enjoy the amazing views from the top and take in a sunset if you can. So gorgeous!
It's posted, but please do NOT go down to the lake from there.
Every year several very foolish people have heart attacks coming back up and jeopardize the safety of our local EMS folks who have to rescue them. It is a far more grueling up hill climb than it looks. I cringe every time the Northflight chopper flies over my house, knowing it has happened again.
If you want a real workout, climb THE dune at the dune climb area. Running back down feels like flying, (but make sure you don't do a face plant.
If you like biking, the Heritage Trail is paved and has lots of great riding miles. Go to Glen Haven for history. I could go on for ages. It has such a variety of activities to offer. You will only have time to see a fraction in a few days. It would be easy to spend 2 or more weeks at Sleeping Bear alone.
Check out Cherry Republic in Glen Arbor for dried cherries and tons of unique and yummy goodies. (Full disclosure, DH grows a lot of the cherries that go into their products.)
Canoeing the Platte River is also tons of fun. Take a picnic to enjoy while you float. It's a pretty easy paddle, except for Loon Lake if its windy. Please respect the endangered Piping Plovers who are currently nesting at the mouth of the river. Dogs are a particular threat, so please keep your fur baby on a leash if you have one.
If you are looking for a casual dinner out, The Cherry Hut in Beulah is a throwback to the 60s ish era. Love their chicken pot pies. For Mexican, the Road House in Benzonia is a local favorite (be prepared for a wait). The ribs at Dinghy's in Frankfort are awesome. I like the dry rubbed, but DH is a wet fan, so have your pick. Fusion has some excellent Asian meals with a view of the water.
And for full on family fun, check out the Cherry Bowl drive-in in Honor. Yes, we still have one. (I think Toy Story 4 and X-men may be coming up, check for sure.)
Gwen Frostic studios and print shop is an interesting stop. Although Gwen passed on several years ago, her art work is being kept alive and her studio was just named to the register of historic places. A good place to pick up some small, unique items for birthday or holiday gifts.
Hope you have an awesome trip. We love it here in the Mitten state!