For fresh water swimming, Akerselva is considered clean north of Nydalen. The best spot may be at Frysja (bus 22-25-54 to Stillatorget). Don't forget that many of the lakes in "Oslomarka" are for swimming and recreation, but a couple of the major lakes (Maridalsvannet in particular) are drinking water for the city. Buy a map and bring your swimming gear!
* For salt water swimming, there are plenty of islands perfect for relaxing and swimming. Ferries depart from Vippetangen and Aker Brygge in the city centre. There are beaches at Huk, Bygdøy as well (Bus 30 to Bygdøy), as well as Katten and Ingierstrand. A new harbour bath opened in 2015 in Sørenga, the new development just south-east of the Oslo S/Opera-area.
* Visit the local city district Grünerløkka, (tram 11, 12 and 13 to Schous plass, Olaf Ryes plass or Birkelunden, tram 17 or bus 30-31 to Heimdalsgata or Birkelunden, or bus 21 to Sannergata). A part of the city filled with cafés, bars, small fashion and designer shops and nice parks. The river Akerselva runs on the west side, with a selection of (well hidden) bars, clubs and cafés nearby. The food market Mathallen, as well as the new Vulkan development with hotels, shops, cafes and offices is situated just across the river from Grünerløkka.
* Take walks or a bicycle ride in Oslo's many forests. Nice gateways to the beautiful nature are T-bane to Frognerseteren (line 1), Sognsvann (line 6), Romsås (line 5), Bogerud (line 3), Skullerud (line 3) or Ellingsrudåsen (line 2), or the train to Movatn or Snippen (local trains towards Hakadal, Roa, Jaren and Gjøvik). Also bus 56 from Storo (tram 11, 12, 13 and T-bane) to Solemskogen or bus 51 from Nydalen (bus 37, T-bane 4, 5 and 6) to Maridalen offer fantastic possibilites for walks and rural life, everything less than half an hour from the city centre. Bring something to drink and a chocolate bar (and do a bit of planning, buses 51 and 56 and the train does seldom run more than once every half hour, less in weekends). Be prepared for some muddy sections of the trail as they take you through some pretty thickly wooded areas. The forest is dense enough that any damp areas of the trail don't dry out. This isn't helped by the multitude of small run-offs flowing down the mountain.
* Biking: Discover the city and its major tourist sites by bike, from April through October.
* The area around Holmenkollen (T-bane 1) is well suited for cross-country skiing, but also a nice area for hiking during summertime. While you are there, you can also visit the ski-museum which is located close to the ski-jump. For additional fun, buy a cheap sledge or "akebrett", or rent a proper one from Frognerseteren, and sleigh down from Frognerseteren to Midtstuen station. This was actually the venue of the sleighing competitions in the 1952 Winter Olympics, and the hill is referred to as "Korketrekkeren" (the corkscrew). If you buy a day-card for the subway, you can race all day long, but make sure you are well dressed, as you are going to get wet.
