Getting Around Boston
Trolleys
Boston has a number of "trolleys" (buses made to look like trolleys) that can be a fun way to get around the city. Typically, the trolleys do a circle around key tourist sites in Boston (a couple also go out to Cambridge); you buy a day pass that lets you get off and on as often as you like (around $20).
Boston has an extensive subway and bus system that can get you to almost every tourist location relatively quickly and safely (although be aware that trains and buses will be crowded during rush hour). Locals call the subway "the T" - you can find the stops by looking for large black T's in circles.
The T's Boston Visitor Pass give you unlimited subway access for $7.50 a day; $18 for three days or $35 for a week. One drawback of the local subway, at least for bar-hoppers, is its hours: The last trains leave downtown around 12:30 a.m. (even though bars in Boston can stay open until 2).
The MBTA's commuter rail can get you to more remote tourist attractions, such as Concord, Rockport and even Providence, R.I. However, on some lines, schedules are particularly geared to getting commuters to and from Boston (i.e., trains out of Boston will be far between in the morning and trains to Boston will be infrequent in the afternoon), so plan accordingly.