Yep, there's usually snow in December in Boston! But this past year was warm, so I'm not sure if there was.
Hmm, I don't remember what stuff is called, but there's lots of historical stuff. There's the Freedom Trail, which is a trail that goes throughout Boston to all the historical stuff.
And have you ever read 'Make Way For Ducklings'? I can't remember the author, but it's a children's book. It takes place in Boston, so at the pond (don't remember the name of the pond) there are swan boat rides and you can see the ducks. Not sure if this is still going on in December, probably depends on the weather.
If you do visit the pond, watch out for the geese. Don't feed them, not even once. If you do, they will bother you for more and won't stop. This happened to Godric, it's rather funny looking back on it!