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 services to these islands in addition to their regular commuter routes.
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.
Further out the fjord is a restaurant called Villa Malla, the perfect spot for a sunny day. I am surprised every time by how long the boat takes (1.5 hrs) but love seeing the lovely small island communities on the way out. The restaurant is almost like being in the tropics, with a large terrace serving a lunch and dinner buffet under some suspiciously green palm trees. They also have a beach and water sports you can sign up for.
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!