I have stayed at the hostel several times with the Girl Scouts and it is truly a hidden gem. There are 2 buildings: one larger main building with dorm style beds and another smaller one with fewer beds and also accomodation for a family. The main building has a large kitchen , large dining room/common area and smaller room for quiet reading, etc. Outside are picnic tables and a nice large yard to relax or play in. Plenty of bike racks to accomodate those arriving by rented bikes. Best of all is the place is a short walk from Surfside beach. At night, with the windows open, you can hear the ocean and see the light from the nearby lighthouse. Incredibly relaxing and refreshing.
You can bring your own food to store and prepare in the kitchen. Or, grab a taxi and head downtown to the restaurants.
Be aware that they will most likely ask you to do a small housekeeping chore before you checkout. But it's a small price to pay in exchange for spending the night within the sounds of crashing waves on Nantucket. Grab your bike and go!