Are you going to visit the Munch museum? Do not return to the city centre immediately after your visit. Instead you should spend some time to explore the area around .
Follow this way , and suddenly you will be in a totally different part of Oslo, called Kampen. Since this area didn’t become a part of the city before in 1878 it is mainly dominated of small, charming, old wooden houses and narrow streets. This is a nice place just to walk around, and do nothing. What you actually can do is to enjoy a coffee at Javelin in Bøgata.
Kampeparken is situated at the highest point, and in periods without too much vegetation, you can have a great view over the city and the hills surrounding Oslo.
Are you really hungry and want a decent lunch, you can pop into Dileks. This is a relaxed cafeteria, not fancy at all, but has probably one of the best eggs and fried potatoes portions in the whole city. They have a vide range of lunches, included vegetarian offers, and for approximately 100,-NOK you will get enough food to survive for many hours:)
Kampen was earlier a workers area, and many years ago this area apparently had so many communists that it from time to time was called Petrograd. Back in 1878 it was a battle between workers and police/military soldiers up here, later on known as Onsumslaget, named after the fabric owner who was the main reason for the riot.
Today Kampen is a very popular residential area, but has still remained some of its earlier atmosphere and it is very different from what you can see in the western parts of Oslo.