Pemaquid Lighthouse is a must see.  There are a couple of flat loops to walk on either side of the lighthouse for easy walks.

Plummer Preserve is a great 2 mile hike with views of the Damariscotta and a cove, and maybe some seals.

Sela Cove is very near Plummer Preserve and similar.  Only 1.2 mile loop vs. 2 mile at Plummer, and the terrain is a bit gentler. Similar beautiful views of the river and cove

Laverna is over 2 miles, especially if you take the red TIbbets trail loop (which we recommend), but the view and rocks are spectacular.  It is a bit of a scramble to get onto the rocks to look out on the sea, but worth it.