Often we go to Shannon Beach ("Sandy Beach"), on the Mystic Lakes:

They test bacteria levels, and when they get high they close the beach. Check the DCR Alerts Page before going.

This is the closer option for us, but when it's closed we often go Revere Beach:

This is on the ocean, but it's sheltered enough that waves are typically just a few inches. It's also colder: I'll swim at Sandy Beach, especially in the late summer, but I'd need to go hundreds of miles south of Boston if I wanted ocean water that didn't chill me too quickly to be fun.

Both have free parking, and Revere Beach is on the Blue Line. Neither have lifeguards: at Sandy Beach there are ropes that keep the older two within an area they can easily touch and we stay close to the toddler; at Revere Beach if they want to go past their knees they need an adult with them.

The older two often want to eat quickly just before leaving or in the car because (a) they prefer hot food and (b) they want to play the whole time we're there. But it's still important to them that the trip be a picnic, and Julia and I enjoy eating on the beach. Also a good fit for portable ice cream!