Grefsenkollen is a beautiful hiking spot in Oslo that offers stunning views of the city and fjord. The hike is relatively short at 3.1 km each way from the bus stop at Grefsenkollveien. It's also possible to drive all the way to the top.
At the top, you can take a break at the historic Grefsenkollen Restaurant, grab a bite to eat, and soak in the amazing scenery overlooking Oslo. If you want something extra special I recommend to book a fire pan with prepared food, stick bread, marshmallows and other snacks. It's really cozy, especially for sunset.
Remember spikes!