September is a great time to head to Maine! The later in September the closer you'll get to some fall foliage, especially around Boothbay Harbor region. If you go there, take one of the lighthouse cruises, Kennebec River cruise my fav, by Captain Fish's Cruises.
Head up north and you'll find lots of roads that head down to peninsulas along the rocky coast. Damariscotta region, check out Pemaquid Point lighthouse with the most unique rock formations (see image below). If you head to the rocky coast for lighthouses, crashing waves sometimes an hour before and hour after high tide, but especially after a storm has gone out to sea.
Some of the best coastal "normal" food you'll find are off the main tourist areas, like general stores where they are mostly privately owned and the owner's take their recipe's seriously, including best lobster rolls (never ask for hot lobster on a roll, always cold).
Get out to one of the islands by taking the Maine State ferry, no frills, or screaming babies as on a tour boat, and they're cheap.
If you have a few hours, go sailing or windjamming on an authentic schooner, out of Portland, Rockland, Camden, or even bar Harbor among other destinations.
I created a lighthouse tourism website (always a work in progress) with attractions nearby if you want to get some info at https://www.nelights.com/exploring/exploring.html
Also wrote a couple more in-depth lighthouse tourism books that might be useful.
https://www.nelights.com/about/lighthouse-books.html
Hope this helps and enjoy your journey!