The Langholmens Vandrarhem (Youth Hostel) is located on an island (you can get there by foot) in Stockholm. It is an old prison. A very green area with even a small beach. Barbecue is possible. If you walk there from Central Station, calculate 30 minutes - it is a nice walk along the water. I definitely can recommend this hostel if you stay some more days in Stockholm. Pay attention to extra fees like sheets, etc.
Prices and more information (exact details), as well as photos, can be found on the booking page (right button).