An ideal place to spend with friends or family. Things you can do: fishing, kite flying, swimming in the sea or just lying on your beach chair to get a tan - if its a sunny day. Changing rooms are there.
You take a ferry from Boston Harbor for $15 a head return ticket. The ferry takes nearly 20 minutes each trip.
Carry your food, water and beer. Swim trunks, towels if you are fond of being on the beach else there if you like to explore the winding paths in the island you are most welcome to do so.
If you have a portable BBQ, you can find a corner away from the hawk eyed marshals/rangers scouting the place and cook your own meats.
Fishing is also an option - carry your tackle. If you have young children in the group - I would suggest kite flying - children love it - carry your kites.
If you are located in Boston - you can even go to the island by your own boat.
Remember - you have to pick up all your garbage when you leave - the rangers will give you free bags for collecting the same. And go early - last ferry to return is around 6 pm
Summing it up - a lovely place to spend an entire day with family of friends.