One thing about being up north, is that distances are a lot further than they look on a map. So you might want to keep that in mind.
Traverse City has by far the most things to do, but can get crowded in the summer. Its a tourist trap after all. There are also some shore diving as well as the wrecks. The shore diving didn't sound too appealing, but I dive in quaries so what do I have to talk about. At least the visibility looked better.
Traverse City also has a neat little zoo, not far from the dunes, and actually has beaches. If you go there I would suggest making reservations early.
Mackinaw is very much a tourist trap, but the island is kind of neat.
Alpena has some very nice diving, but as far as I know its all boat diving. The town is very laid back, but there is some shopping and the favorite occupation seems to be relaxing.
Unless you like really cold water, you might want to consider July or August, unfortunately it seems that the vis drops from algae blooms. And if you are going to Superior, they define cold water and one very long drive. But very scenic.
Bob