There are several waterfalls and nice pastoral scenes on the island, but my best memories are from exploring the Northern Channel islands and coast line by boat. You can rent a small boat in Little Current or Manitowaning (and in many other places on the northern side of the island - stick to the north east end). There are many small inns and BB's on the island, and compared to 30-40 years ago when many American families came there for holidays and it was difficult to find some accommodation, nowadays there are always some vacancies in most places throughout the summer.
The landscape forms in the North Channel are similar to the northern Georgian Bay scenes, yet different. Bigger water, and not as many shoals.
If you like to explore the coastal areas from the water, another beautiful boat trip (allocate full day) is to Baie Fine just east of Manitoulin.
I would recommend to rent a small boat near White Falls on the mainland, just north of Little Current and head for a long and deep Baie Fine with spectacular quartz mountains on the north side, which ends at Killarney park. Do it only in calm weather since you'll have to cross a fairly wide expanse of open water before reaching the protected waters of Baie Fine.