Oslo is at the end of a long fjord, with lots of islands of varying sizes. In fact, many Oslo locals have their summer cabins on these islands so that they can easily escape to their summer retreat. The public transport company in Oslo, “Ruter”, runs regular summer services to these islands in addition to their regular commuter routes which run all year round.
The boats go from the pier at Aker Brygge, close to the town hall, and operate on the same ticket system as the bus and tram so it’s very handy to buy them through the app (your period ticket is also valid if you bought one of those). The closest island is Hovedøya (main island) which is not inhabited but perfect for a walk and a bite to eat in the café. In the summer I like swimming from the beaches on the western side.
During winter you really get a feel for how exposed to the elements Oslo is and how cold and grey the sea can be.
Being on the water gives you a very different view of the city and the local commuter boats give you the experience on the cheap!