If your in-laws haven't been to Mackinac Island, I would recommend the ferry ride to the island. It's the only place I've been to that you can walk your buns off and still gain weight!
Too many fudge shops....you can gain weight just by BREATHING!
The BEST FUDGE I've ever had!!
I'd also recommend the carriage tour. It is the best and most relaxing way to see the island and get a lot of the history of the island and area (all of the drivers are well versed in that area, along with some humor...I was a driver in the early 70's for a summer season). Plus it would be 'easy' on the in-laws. If they are up for a walk(all down hill from the Fort back to town) the carriage tour will let you off at Fort Mackinac for more history and some pretty good views. Oh yeah, don't forget to tip your driver
if he/she was worth their salt....the pay was very low and most rely on the tips to help with college. That part probably hasn't changed in all these years.
Anyway, if you don't stop at the fort, the carriage will drop you off back in town.
All and all, it makes for nice day of sightseeing.