Owned by Harvard University, The Arnold Arboretum is a wonderful place for stress reduction. They care for a wide variety of trees here (from all around the world), and there is a wide paved walkway through the place. Not as "wild & wooly" as the Audubon places or Middlesex Fells, but a very nice stroll, and a place to take nature lovers who might not want to do serious hiking.
The Charles River is a large stretch of nature, with paths, bike trails and wooded areas (plus a filthy river, but they're trying to clean it up). See this information about the Charles River and activities. To give you an idea of just how dirty the water is, I've noticed that when it rains, the ducks and Canadian geese far prefer the puddles in the parking lot to the river itself. Of special note: The Soldier's Field Road Parking Lot Bird Contingent. I have no idea why, but the birds *always* hang out in the parking lot near the Publick Theatre, all facing Southwest, whether the sun is out or not. Why is this? Does anyone know? Is it a sacred Indian burial site? Energy vortex power spot?
The Middlesex Fells Reservation is an amazing vast expanse of wilderness, reservoirs, lakes, mountain bike trails, and lots of great bird watching — overlooking the lovely scenery of route 93 and beautiful downtown Medford. This nature preserve stretches over five towns: Medford, Malden, Stoneham, Winchester, and Melrose.
Massachusetts Audubon, where the hippest birds hang out. Also, a terrific place to wander around in the woods and forget about the modern world. You'll probably want to check out the Massachusetts Audubon Society home page, but here's a little bit about their Boston area places:
It's not just for dead people. Mt. Auburn Cemetary, 580 Mount Auburn Street, Cambridge, MA (617) 547-7105. Hours: Mon. - Sun. 8 a.m. - 5 p.m.; Daylight Savings Time until 7 p.m. A wonderful place to walk around, look at bird, trees, some cool old graves (lots of Celtic crosses), and mausoleums. Stop by the main entrance and get a listing of famous people buried there, and a directory of different trees from all around the world. They also have a tower, which has a great view of Boston and surrounding areas, and an awesome sphinx (Memorial to the Preservation of the Union). A few very nice ponds with ducks and geese. Mary Baker Eddy of Christian Science fame is buried there, as is Henry W. Longfellow, Charles Sumner, and Charles Bulfinch. Be warned: no bicycles or rollerbladers (be respectful, people). Serious birders frequent there, and there's a board near the front gate where you can record your special spottings (I've seen red-tailed hawks there; very cool). Visit the Mt. Auburn Cemetery official site.