Oslo's beloved woodland sits to the north of the Holmenkollen Ski Jump and is a prime destination for hiking, mountain biking, sledding and skiing. In the summer, the Tryvannstårnet observation tower is a good place to start a hike or a bike trip; or head to the Oslo Vinterpark for skiing in winter. From the Holmenkollen T-bane station, take the scenic ride to the end of the line at Frognerseteren and look for the signposted walking route.
It's also the geographical centre of the city, which must make Oslo, quite appropriately, about the only capital in the world to have a wild forest at its heart. Make sure you take a container for picking blueberries in summer.