Peaks Island has 12 miles of trails, on paved and dirt road and through woods and marshes. A little hike is the best way to see many of the sights. Portland Trails and the Peaks Island Environmental Action Team have published some trails.
The Peaks Island “Loop:” https://www.trails.org/trail/peaks-island-loop/
PEAT Naturalist Tour: https://greenerpeaks.org/naturalist-tour
You could also check out the Wander Peaks Island Audio Tour: www.wanderaudiotours.com