South of Boston is Nantasket Beach in Hull. Definitely not a secret, but you need to check a surf report because it's not always breaking. Luminate Surf Shop just opened a store there.
Deff worth a look at Nantasket if there are waves at all. Its a fun beach with enough room to spread out, and no one will hassle you here, its no secret spot. Luminate is awesome they have another shop in Marshfield, MA and the owner is an awesome dude, super friendly.