Not Maine, but Portsmouth, NH, near the old Pease Airforce Base, is Great Bay National Wildlife Refuge. There are some really neat walking trails/boardwalks that are easy to access and there are some interesting things to see.
https://www.fws.gov/refuge/great-bay (If you pull up the map, the walking trails are the dotted lines near the North Apron.)
https://www.dailykos.com/stories/20...en-History-Pease-Air-Force-Base-New-Hampshire
Strawbery Banke in Portsmouth. It's a collection of colonial-era homes and shops from old-time Portsmouth. We all enjoyed walking through the buildings, listening to the docents, and learning about the colonial roots of the area.
There is also a dry-docked submarine, The USS Albacore, at Albacore Park in Portsmouth.
Mt. Agamenticus in York. You can drive up to the top of the mountain for some great views. There are picnic areas and mostly-easy hiking trails. I can remember going here as a little girl in the 1960s, with my grandfather, and going up all the stairs to the game warden's fire watchtower. We also used to ski here, when it was a "resort" and you can still see the bases and anchorages for some of the ski lifts.
https://agamenticus.org/
Nubble Light in York. The lighthouse is pretty, even though you can't go over to the island or in the light anymore (unless you know where the secret tunnels, up through the rock wall and only accessible at low tide, might be

). Fox's restaurant/take out is right there, and Dunne's Ice Cream, the best in the area, is just down the road. It's also a lovely walk along Long Sands Beach.