One of the most interesting things I found on my search for 101 things to do in Boston was a suburban hike through the Sudbury/Cochituate Aqeuducts in Newton, Massachusetts (accessible via the green line). Back in the day, a tributary of the Sudbury river was dammed to form an artificial lake, Lake Cochituate. The aqueduct was created to funnel water from Cochituate to the Brookline Reservoir. Brookline Reservoir actually fed directly into the Boston water system, and apparently the ceremony to celebrate the first water from Lake Cochituate flowing into Beacon Hill in 1848 garnered quite the crowd!
Wikipedia (the darkened stripe is the watershed/aqueduct area).
Neither of the aqueducts are in use anymore, becoming both green spaces and sewer lines (which is why some of the aqueduct is underground and some of it was still ravines cut into the ground).
Posted here, this loop traverses people's backyards (stepping stones often provided), and a park, winds along a Whole Foods, and we walked under trees and through meadows, and next to a marshy bog. Although we missed the fall colors, a long walk with crunchy leaves underfoot, eating honeycrisp apples on the way, watching the trains go by at Eliot, and conquering the most marvelous sledding hill - it was a great way to spend an extra-balmy November afternoon.