The days are shortening and cooling, but the islands in the Oslofjord are still welcoming. You actually have have several possibilities (Check the Gressholmen article for example). However, the closest island, and the one I find myself visiting most often is Hovedøya. Just a short 5 minute ferry ride on the number 92 or 93 boats from Vippetangen, Hovedøya is the main land body visible from the harbor.
The most notable site on the island are some monastery ruins dating back to the 1100s. They’re just a short walk from the dock and are fully open to be walked through and explored. My typical Hovedøya visit will start with a walk through these ruins, followed by a wandering hike through the trails and water’s edge. Then, depending on the weather I might go for a swim or a picnic lunch. You aren’t allowed to build a fire on this island, but grilling is a popular alternative, and combined with a large open sportsfield you’ll have plenty of possibilities for spending an afternoon.
Hovedøya | Relaxing
24 hours daily | Depending on Ferry schedule