Catch a baseball game at the oldest active ballpark in MLB. Make sure you get a fenway frank and for the love of god do not wear a yankees shirt :)
Combination of nice and touristy shops, restaurants and street performers. Boston's promenade if you will.
Boston's Little Italy. Dare I say the best italian food you will ever eat. All the restaurants/pastry shops are top notch. But the following are some of my favorites: Trattoria di Monico, Mike's Pastry's, Modern Pastry's and Bovas
Beautiful parks, photo ops with the "make way for ducklings" sculptures and swan boat rides
Lots of high end shops and restaurants and location of the Boston Marathon finish line!
I know I know who wants to visit a library but this one is incredibly beautiful!!
Quintessential Boston... beautiful brownstones, high end shops and amazing restaurants
If you want to stay in Boston I recommend staying in this area as it is very central to all the must sees
Walking/biking path along the Charles River
Really cool rooftop bars and good restaurants
Infamous sightseeing tour in Boston, you'll see them everywhere!! Cruise around Boston and Boston Harbor in these WWII style amphibious vehicles with a sprinkle of history.
Get some authentic New England Clam Chowdah
My go to place for a lobstah roll!
If you are staying on the Northshore, I HIGHLY recommend going to Salem, MA in October. Salem is known for being one of the oldest colonial settlements in Massachusetts and it is where the Salem Witch Trials took place. So it gets CRAZY during October/Halloween. You will find everything from haunted houses, ghost tours, witch museums, hocus pocus tours (see some of the buildings/areas from the film), psychics and the arguably the best people watching you will ever encounter.
Be basic and check out one of the local farms and/or apple orchards for apple picking or corn mazes. Some of my favorites in the area are Russell Orchards, Marini Farm, Cider Hill Farm, and Applecrest Farm Orchard (this one is new hampshire but not far).