I would go to St Ignace and take the ferry from there as it's a more scenic way. Check with the ferry companies as one of them takes a little side trip on the way over and goes over to the bridge and then out to the island. You can take your bikes or rent bikes or take a buggy ride around the island. I 75 is fine if you want to just get there , if you want a more scenic route go over to 23 and along Huron. Don't forget to climb Castle Rock when in St Ignace. The Pink Pony has great reuben sandwiches and long island ice tea on the island.
Now not to mess up your plans but if you want to travel farther then that I would probably spend the time in the Pictured Rocks area.
