The main attraction at Isle Royale is simply the wilderness. Most notable on the wildlife front are park's populations of both moose and wolves. Though numbers vary from season to season--animals sometimes travel over the frozen lake into Canada--there are on average about 1,000 moose to spot. Dogs aren't allowed anywhere in the park to prevent canine disease from jumping to the protected wolf population.
There are only two truly developed areas in the park. The first is Windigo, located in the southwest end of the island. It offers campsites, showers and a boat dock. Rock Harbor, the other option, includes the same facilities and also has a lodge and restaurant; find it at the northeast end of the island.
Camping is your best bet, since day trips are limited by the transportation schedule: Storms or other inclement weather can put a delay in your trip, and you'll spend more time traveling to the island rather than enjoying it.
Now is really the best time to visit because Isle Royale is the only national park that closes down completely at the end of the season.