If you're heading to Maine in September and are looking for something to do... Plan on being there on September 13th, it's "Maine Open Lighthouse Day" 27 of Maine's 62 Lighthouses are open to the public. Some of the foundations throw a great party with a "Down East" cookout, others... well, there's the lighthouse, enjoy.
I've been participating for years and for me the mid-coast is the best. We stay at Durham KOA, just outside of Freeport. Hit Pemaquid Point Lighthouse in Bristol first, then Owls Head in Rockland and cross the harbor and to Rockland Breakwater light. Pemaquid throws the best party and it's free too.
Last year we went to Acadia NP, stayed Oceanside KOA, then did Bass Harbor Head Lighthouse, Marshal Point and took a boat ride to get 2 more light form the water.
The problem is so many lighthouses so little time... Do it once and you're hooked... I have a Lighthouse Log book with all the east coast lighthouses in it and we get a stamp or something from each one we visit. I was reviewing my logbook and was surprised to see we've been 73 lighthouses since we started. Now, I find myself planning my trips around Forts, Lighthouses and boats... Next is Old Mystic Seaport... A lighthouse and the Whaling ship.
http://www.lighthousefoundation.org/openlighthouseday.htm