The Oslofjord has around 40 islands within easy ferry reach of the city, several of them popular for swimming, hiking, and summer camping. The closest and most accessible are the Hovedøya (15 minutes from Aker Brygge), with the ruins of a 12th-century Cistercian monastery and a shingle beach; Gressholmen (20 minutes), car-free and good for cycling; and Nakholmen (25 minutes), with sandy beaches.
Ferries depart from the Aker Brygge terminal (boat 92 for the inner islands); all use the Ruter card (same system as the city's buses and metro). The islands are particularly busy on warm summer weekends when half of Oslo retreats to the water. Come on a weekday for a more peaceful experience. Swimming is good — the fjord water is clean and the Norwegians are enthusiastic outdoor swimmers well into October.