Although Boston has many beaches, all swimmable, many people just go to the Cape and don’t think to try the beaches nearby. This link from BU TODAY, To the Sea by the T has a really cool Map showing the beaches and how to get there via the T for the cost of a subway fare. My favorite is Revere Beach (200 meters from the Blue Line Revere Beach stop), particularly for the National Sand Sculpting Festival that runs July 19-21 in 2013. Most people’s favorite is Nantasket Beach on Hull (best by car, since still a 3.1 mile walk or short taxi from the T). Bicycling is another option… Taking the ferry to one of the Boston Harbor Islands is another easy choice.
A guide to MBTA-accessible beaches
Unbelievable pictures from past sand sculpture contests at Revere Beach.