Langøyene is an island in the innermost Oslo Fjord. It can be reached by ferry during the entire summer season.
And why reach it? Mostly for the free tent camping. You can stay and camp with no limitations in time, as long as you follow the rules of the Oslo municipality.
They include keeping the camp tidy and quiet between 23:00 and 7:00, keep at least 3 meters distance between the tents, and avoid lightnig fires.