If you are going to look at other parts of Maine, you might think about a visit to the Moosehead Lake area. August will be moose season and you might get some good images.
Boat trips - there is one in Portland that will let you see some of the harbor lights from the water. Not a real long trip, and they go several times per day
Casco Bay Cruise . They say you see the twin lights, but you only will if you have a real long lens. They don't do past Portland Head unless they have changed the route (and the 90 min length would say they haven't). Still worth the time, hopefully they will have a new person narrating (he never stopped talking . . .). Very reasonable at $18.
There was a company that did a lighthouse cruise of the Kennebec River. You can check to see if they are still running, but their website hasn't been updated for quite some time.
Long Reach Cruises .
Monhegan Boat Line is another possibility for a cruise to pick up a few lights from the water.
Maine Maritime Museum in Bath might have a cruise of some type. They used to allow the Long Reach people to use their dock as I recall.
Of course, I would also check with the folks at the
Maine Lighthouse Museum for more ideas.
I'll look forward to your planning and trip blog.