Garden in the Woods, Framingham, and the Arnold Arboretum,
Jamaica Plain.
I'm sure there are many more ...

Such a short visit. So much to see. An often overlooked treasure is the Mt. Auburn Cemetary in Cambridge. A taped guided tour is available, either walking or driving. This would also be your chance to see the world famous Glass Flower exhibit. An oooh-aaah experience. It used to be at the Peabody Museum in Cambridge, but I understand it has been moved to another location. Ask at the hotel for directions.

Be sure to walk in the Cambridge area with your camera. There are some fantastic home gardens there.
Coleen

The Public Garden, with its world-famous Swan Boats, is very popular -- as is Boston Common with its Frog Pond. A walk down Commonwealth Avenue to the Arlington Street entrance to the Public Garden will reveal many lovely front-yard gardens.

The Museum of Fine Arts (MFA) on Huntington Avenue has a lovely Japanese Garden by the West Wing entrance. Nearby is the Isabella Stewart Gardner Museum with its incredible Italian courtyard garden. From both museums you can walk to the Victory Gardens (community gardens) and the Rose Garden in The Fenway/Park Drive area (best done in the daytime; maybe ask for directions at the MFA information desk).

If you can visit Brookline (Route 9 West from Boston), there's the Frederick Law Olmsted National Historic Site.

(note: copied from other post I've written)
(also: Rose Garden is supposed to be closing for refurbishing. Not sure if it has begun.)

Enjoy!