Start with a walk on the roof of the new Opera in Oslo designed by Snøhetta architects. It has proven very popular with the locals and visitors alike. You will find the entrance to the right outside the Oslo Central Station on the side facing the harbor across a bridge. With the snow and ice of winter Oslo it is probably slippery though so be very careful; you enter at your own risk. Perhaps enjoy lunch inside the Opera after.
With the Holmenkollen skijump currently being demolished to make room for the new one, I would suggest an afternoon visit to the newly restored cafe/restaurant Grefsenkollen. At 380 meters above sea level it has a magnificent view of Oslo, the fjord and a great sunset weather permitting. The restaurant has good food and there is a sporty crowd on the weekends due to the nearby ski areas. http://www.grefsenkollen.no/ (in Norwegian only)
Bus 56 towards Solemskogen to Trollveiskrysset, then a 15 minute walk. Bus all the way to the top on the weekends.