For years my wife and I would go to Mount Desert Island right after school opened as that was when most of the people "from away" were away again. Now the cruise ships come in followed by "leaf gawking season" , so the fall is not always less congested. However, the air is crisp, it tends to be less crowded than July and August, the prices drop a little, but the island is just as beautiful as during prime season. For the first time visitor here are a few secrets to look for other than down town Bar Harbor, whale watching and such. (1) drive to Bass Harbor eat at the lobster pound and visit the lighthouse, (2) drive to Wanderland and hike the 1/2 mile trail to the ocean, you may be the only one there, (3) have a popover at the Jordan Pond House, but first walk around the pond...about two hours to build up an appetite.
Stop at the Visitors Center, rent some bikes are ride the carriage trails. Drive to Southwest Harbor and tour the (for free) Astico Gardens and have a lobster roll on the porch overlooking the harbor. Take the road less traveled it will make all the difference.