By Other Means
Although pedestrians, bicycles, and snowmobiles are not permitted on the Mackinac Bridge, the bridge authority will take you across, for $2 per person and per bicycle, or 8AM-8PM only $10 for each snowmobile and rider.
By car
From the Upper Peninsula, you have to cross the Mackinac Bridge, via St. Ignace. The "Mighty Mac" a certain burger vendor objects to another nickname is 5 miles long and at its highest, 200 feet from the surface of the Straits of Mackinac. The toll is $1.25/axle for passenger vehicles that's $2.50 for a car or motorcycle, $2/axle for motor homes, and $3/axle for anything else. The drive can be a little intimidating, but only two vehicles have gone over the edge in its half century of use: one a lightweight Yugo caught by very high winds, the other an SUV in a suicide; both were speeding. If you're uncomfortable driving across, a bridge authority employee will happily drive your vehicle for you including motorcycles--they love Harleys. Just ask the toll collector. The bridge closes to traffic when the winds exceed 65mph, but is otherwise open all the time.
The car ferry service that connected the two peninsulas before the Bridge was built is no longer available.