Hovedøya Island is located closest to the Aker Brygge wharf. On the west side of the island you will find two nice, large beaches. The large grassy area in the middle of the island is ideal for ball games and other fun outdoor activities. If you walk south, you can see traces of Norwegian military history. Two large gun emplacements dating from 1808 and two gunpowder depots remain from the period when the Norwegian Armed Forces used the island as a military facility. Flowers, trees, animals, and anything else you might come across out there are protected, and parts of the island are actually a nature reserve.
Langøyene has a large beach, and there is also a beach for naturists on the southern tip of the island – for visitors who like to enjoy the summer without wearing a stitch. There are also football fields, sand volleyball courts and a kiosk for basic provisions. If one day is not enough, you can spend the night in a tent on North Langøy.
Gressholmen have great swimming spots as well, but if you want to do some exploring, you can walk to Heggholmen Lighthouse, one of the oldest in inner Oslo Fjord. This is also where Norway’s first main airport was established in 1927, and it maintained its status until Fornebu Airport assumed the role in 1939.
Lindøya have two beaches and there is also a swimming stadium dating from 1949, and swimming competitions are still held there. There is both a small shop and football field in the middle of the island.
Story sourced from: https://www.visitnorway.com/media/news-from-norway/now-is-the-time-for-island-hopping-in-inner-oslofjord/?lang=uk