Managed by the Massachusetts Bay Transportation System (MBTA,) Boston's metro is comprised of myriad bus routes and the Blue, Green, Orange and Red rail lines. Known as The T, the system is a breeze to use. Depending on the length of your stay, it is more economical and practical to purchase a 7-day pass rather than paying ad-hoc every time you board a bus and / or use the rail lines. There are a number of options available on the MBTA's website for fare options.
The Green Line, which starts in the north and then travels east with 2 branches to the south, is one of the easiest way to see the downtown part of Boston. One could start at the North Station, for example, and conclude at Longwood in a single day, exiting at each stop along the way to explore the significant sites and then re-boarding to continue your journey. Here is a list of those stops and the places worthy of your time.
North Station :
― Citizen M Hotel Boston
― Little Italy
― Old North Church
― Old North Neighbourhood
― Leonard P. Zakim Bunker Hill Memorial Bridge
Government Center :
― City Hall
― City Hall Plaza
― Faneuil Hall
― Quincy Market
Park Street :
― Boston Athenaeum
― Boston Common
― Massachusetts State House
― Park Street Church
Copley Square :
― John Hancock Tower
― Old South Church
― Restaurants and shops
― Trinity Church
Prudential :
― Observation Deck
― Prudential Building
― Prudential Mall
Symphony :
― Boston Philharmonic
― Boston Symphony Orchestra
― Northeastern University
Museum of Fine Arts
Northeastern University
Longwood