The following is what I was able to accomplish during my two days in Boston, having never been there before. After my experience, I would recommend three to four days.
Massachusetts State House
Boston Public Library
This may not seem like much, but it’s easy to spend an entire day walking around Harvard’s campus. It’s a very big, beautiful place, as is the Boston Public Library. There are also many historic things to see along the Freedom Trail. You can see the original Cheers restaurant. I did not eat there, however, because I found the menu to be too expensive. Unfortunately, I did not have enough time to visit the Paul Revere House or the Boston Tea Party Museum. Quincy Market is all in doors, and contains a large variety of food stalls and souvenir stands.
Also, it should be noted that it was very cold, rainy, and windy my second day in Boston, so I tried to stay in doors as much as possible.