One of my favorite parts of Oslo is that you have this fairly small city situated along the fjord with lots of green hills and recreational areas surrounding it. There are several nice viewpoints in the city which offer opportunities for a nice excursion. My number one view spot is Grefsenkollen.
As a fairly steep but short hill close to the city center, Grefsenkollen offers a variety of opportunities. A natural starting point to hike would be from Storo, which you can reach by all means of transportation. The hike from Storo would take less than 1 hour to the top and there are several ways. Google is your friend. Should physical activity not float your boat, do not be scared off. Grefsenkollen is reachable both by public transport (bus number 56 Nydalen/Grefsenkollen) or by your own car. On the top, there is even a great restaurant and cafe, Grefsenkollen Restaurant, where you can enjoy a nice cup of hot chocolate, a waffle or a bun, or a more advanced dinner option if you feel like it.
As a bonus, not far from the top by the restaurant, close to the main parking lot, there are several amazing “secret” view spots where locals come throughout the year to go camping in their newly bought hammocks taking images for their SoMe profiles. There is a reason for it. The view is great. I find the spots by following the paths in the forest or ask somebody local on the way. Enjoy.