September, especially after Labor Day, is one of the best times to visit Maine. Things slow down from the hectic tourist season, the weather is usually still gorgeous, and everyone seems more relaxed.
Now October you have to watch. Most campgrounds and RV parks close by Columbus Day, some even earlier. If you are drifting into October be sure the places you want to stay will be open. Those that are open on Columbus Day weekend are likely having a last "Hurrah" or leaf peeper crowd, so you may need reservations. Some of the summer businesses will also be closed for the season.
Also be aware it can snow (not heavy, but it might accumulate) in early October in some parts of Maine and temperatures can reach freezing at night. Pack appropriate clothing
Lastly, watch out for the cruise ship dockings in BH
. Some of those over 4000 people into town for the day which can quickly overwhelm venues (Anthem of the Seas, Norweigan Escape, etc.) This also applies to Portland, if you are thinking of exploring that city. Plan your visit around the large ships if you can.