With a 12 year old, I'd emphasize the historical sites. There's something about seeing a site you've read about in history books. Of course, a 12yo may not have the same tolerance for this as I do, so I wouldn't overdo it.
I enjoy seeing some of the older cemeteries in the city.
And I recommend Harpoon Brewery.
"Far more critical than what we know or do not know is what we do not want to know." - Eric Hoffer