Piers Park is a landscaped waterfront park in East Boston, with open views across the harbour to downtown. It has lawns, playground space, walking paths, and a sailing centre, so it works both as a local park and as a visitor stop with one of Boston's better skyline angles.
Come in clear weather, especially late in the day when the light is on the city. It is also a useful reminder that some of Boston's best harbour views are from across the water, not from the downtown side.