When will you be in town?
The Boston Common is lighted up for Christmas at this time of year. Also Fanuel Hall is festive and has lots to see.

Here's what I'd do.
Walk the North End, have a bit of pasta. Then continue (under) the central artery overpass (part of the big dig) into the Fanuel Hall area. Stop by the Union Oyster House for a bit of Lobster Bisque. Stroll around Fanuel Hall, then go up the stairs across Government Center, and take a left onto Tremont Street. That will take you to the Boston Common (lighted for Christmas, pretty view up the hill to the State House). Continue through the Boston Common, cross the street into the Boston Gardens. The Boston Gardens then will exit you at the Ritz Carlton Hotel where, I suppose you could have a spot of afternoon tea. Then continue up the very tony Newbury street where you can window shop. Walk up Newbury and at Exeter St. you can take a left and end up at Copley Plaza, with the Boston Public Library, and behind it is the Copley Plaza Mall.

That should keep you busy.

Have a ball. Boston's a really nice city.