Hey, Boston is the city to be... The trip doesn't complete if you do not get the Duck tour, visit the Fenway Park (stadium), The Aquarium, Fanueil Hall and have food there & see the boston skyline from the charles river (probably take a dinner cruise on charles river)..... Santarpio's Pizza is worth to die for, Fire Ice restuarant, EAT restuarant, I can go on and on... these are some of the places you should have food.. There are so many other places near Mass too see if you have more than couple of days... Have a nice and a fun time here...

How many days will be enough for a trip in Boston? What are the places that are recommended to visit?

nice historical US city, high tech also.... but too cold

You can see most of Boston by bus, subway or foot. Some of the most popular areas are Newbury Street (which has tons of shops), Quincy Market (more shops), the North End ( a lot of the Italian restaurants). Between google maps and the Mass Transit authority (MBTA), you can map out directions to any location.