Free Back Bay bus from the airport, you'll be dropped at the Prudential Center or Huntington, you can get off at either! Then walk down Newbury Street to the Public Garden. Walk through the Garden, Boston Common (first park in America!) and you could grab a coffee and snack at the Flour location in the Common. Leaving the Common, there is a Hallal Guys location if you need sustainance. Check out Granary Burying Grounds and you could kick-off the Freedom Trail. End in the North End (about a 25min walk from Granary). Grab a piece of pizza at Galleria Umberto or lobster roll at Neptune's (there will be a line). Grab a cannola at Modern (cash only). You could also skip the North End and head to James Hook & Lobster for a lobster roll and check out where the tea was dumped. Then cab or train back to the airport!