Trying to incorporate some of your original plan, you could do this:
Day 1: drive from Cinci to Indiana Coast. That's about 4.5 hours of straight driving.
Spend the night there. I have no insight.
Day #2 Indiana Coast to Sleeping Bear Dunes. That's about another 4.5 hours of driving. Pretty things along the way too. Spend some time at the sand. Maybe stay there.
I've been there years ago but have little local knowledge.
Visitors Guide
1.5 hours north will get you to Charlevoix Area so may be that is your final goal for day #2.
OR
Skip Indiana and Sand Dunes and head straight up I-75 to the Charlevoix area on Day #1. It would be an 8 hour drive so leave Cinci early so as to not have to set up your tent in the dark.
Camping locations in Charlevoix Area:
Fisherman's Island State Park in Charlevoix. On the shores of Lake Michigan. Bigger lake can lead to significant amount to wind. Sand beaches but must walk on rocks in the water. Trip Advisor
Young State Park in neighboring Boyne City. Located on inland lake, Lake Charlevoix some much less difficult winds. Not a tiny lake. Lake Charlevoix is roughly 3 miles wide and 14 miles long. Over 150 deep at its deepest but wonderful sand beaches and sand bottom to the lake. Trip Advisor
Whiting County Parkmuch smaller local, county park with camping. Small place but it will likely still be full. You will be able to see Lake Charlevoix from your camp site. Yelp
Petoskey State Park in neighboring city of Petoskey. Trip Advisor
All are good. I have not camped in any in years but I might recommend Whiting as a smaller local vibe. All the other are Michigan State Parks. The Michigan State Parks are very well run. There is a required admission sticker/pass but one pass will bet you into all parks for the day/week/year.
Kayak:
The Jordan River Valley. The Jordan River is one of the tributaries flowing into Lake Charlevoix. The river ends and the lake starts at the City of East Jordan at which point, the lake is known as The South Arm of Lake Charlevoix Through Companies like this you can arange livery service so you can spot your car down stream and only have to maneuver the river along with the current.
Trip Advisor
Informative video
The other tributary to Lake Charlevoix is The Boyne River.
A livery service for Boyne River and a place to call for some local knowledge.
Sample video
When goggling anything about The Boyne River, be aware that there is also a Boyne River in Ireland (the original) which is a much more violent river.
For more rivers, about and hour south there is also the Au Sable River (locally pronounce Awe Saw Ba). This is wider river and very popular. Companies like this will help you spot a car at various lengths. This river is long enough that there is overnight camping available along the way if you want to camp out of a canoe. Located in the area of Grayling, MI. You can goggle a whole lot about The AuSable River.
Sample videos
Sample videos of Canoe Camping day #1
Day 2
You could camp down this way or just come down here for a day trip keeping your base camp in Charlevoix area.
Lake Kayaking:
Certainly Lake Charlevoix.
Many places you can drop in.
For a local vibe and if you can beach launch here is a tip. Find Glenwood Beach Rd in Boyne City This road of cottages dead ends at the water but also at the dead end you come to edge of Young State Park. This is a real "locals" swimming area. Nice sand (mixed with a little grey clay) that is very shallow a long way out but what makes this beach the best is that while the State Park beach will be packed with people, this beach will have maybe one or two other people there even on the busiest holiday weekend. This is fully open to the public and there is some on street parking where the road ends but since it is about 1 mile down a dead end road, only the locals know of its existence.
Here is a picture of The City of Charlevoix, MI.
It boasts that it is a city on 3 lakes.
At the top of the photo you have Lake Charlevoix then a channel to Round Lake and then a draw bridge and channel to Lake Michigan. All of this is within the small city's city limits.
Round Lake is neat harbor. I am not a Kayaker but I think it would be neat to Kayak in the harbor. I must note that this should be done on a week day. On the weekends this harbor gets very busy and a giant power boat could kill you.
I would recommend you put in at Depot Park on Lake Charlevoix. Near the youth sailboats you will find a dock at the park or beach launch. This park is on Lake Charlevoix but right at the mouth of the channel leading to Round Lake. You can the kayak the channel and round lake. Make not to check out Park Island and get a good close look at this boathouse previously owned by the owner of Four Winns Boats
Second video
Another boat house down the street
I can't say I recommend kayaking in Lake Michigan. Waves can be pretty turbulent on the Great Lakes in unprotected water.
I do recommend two other inland lakes in the area. Torch Lake or Walloon Lake.
More later...