Here is a very basic list of other things to see while in Boston: The Boston Public Garden, Faneuil Hall, walk around the North End, and walk around the Seaport.
There are also a lot of breweries in the area that are worth checking out: Sam Adam's Brewery, Night Shift Brewing, Trillium Brewing Company, Harpoon Brewery, and many more.
Even if you don't go see a game or a show there, Hub Hall is underneath which has multiple food options.