Boston is one of the oldest and most walkable US cities. Back Bay, Beacon Hill, the South End, the North End, and Cambridge all combine dense retail, MBTA T access, and park proximity. Boston's compact size makes car-free living practical for most central residents.
Back Bay, Beacon Hill, the South End, the North End, Fenway/Kenmore, and Cambridge (especially Harvard Square, Central Square, Inman Square) are the most walkable.
The MBTA T has 4 subway lines (Red, Orange, Blue, Green) plus extensive commuter rail. Service can be inconsistent, but coverage of central Boston and Cambridge is good. The Red Line covers Cambridge, the Orange Line runs north-south through downtown, the Green Line covers the western suburbs, and the Blue Line covers East Boston and Logan.
Cambridgeport, Inman Square, Central Square, Mid-Cambridge, Somerville (Davis Square especially), Allston, and Brookline all offer reasonable commutes to either campus.