I assume that you already know about Mackinac Island.
Other than the casino, St. Ignace is pretty much limited to Castle Rock, Museum of Ojibwa Culture, and the Father Marquette Memorial. None of those take more than about a half hour to visit.
On the way to the Sault, you could turn east and go through Cedarville to Drumund Island, before going on up. It is a nice drive.
But I would be more inclinded to drive up to Tahquamenon Falls, Whitefish Point (with the Shipwreck Museum and then back down and across with perhaps a stop at Point Iroquois Lighthouse. Depending on your level of intrest, that could take most of a day.
If you take the direct route to the Soo, it is 50 miles of freeway. At the Soo, you have the locks, St. Mary's RIver & Soo Locks boat tours. a side trip on into Canada, the Vally Camp Museum ship and another casino.
There are many more places to go farther west in the UP, but those are the most interesting things that are closest.
Mackinaw City also has a much more to offer on the south side of the Bridge.
Edited: 2:52 pm, April 06, 2010