September is really the best time to visit.
Acadia is really nice, I like there for a summer and there's tons to do. They have Whale watches and interesting little restaurants and shops. Plus there's the national park where you can go hiking, biking and horse back riding. You're right on the ocean so one day you can go hiking up Cadillac Mt and the next day you can go to the beach.
You can go right up rt 1 in September and stop at all the little coastal towns. Route 1 is awful in the summer, though but by mid September most of the tourists have gone home and the kids are in school, so it's not so bad.
If you're more into hiking or skiing (of course you'd have to come in December to March for prime skiing) you can always go to western Maine.
Very pretty, tons of mountains and endless trails.
It just really depends on you want in a vacation. When my In-laws got re-married after their divorce (long story) we purchased them a weekend at the
Grey Haven Inn in Georgetown. It's beautiful, right on the ocean and right down the road from Reid state park, which is really great itself. My in-laws were trilled with the gift and their stay.
Hope you get a chance to visit. Someday, as nice as it is here, I would like to be a Maine tourist rather then a Maine resident.
The snow and cold has really gotten to me this year!