For seeing some of the core pieces of Norway old viking history, Viking ship museum is the perfect location. They have a collection of the large Viking ships Oseberg, Gokstad and Tune. It also hosts founds from the chief grave at Borre in the Vestfold district.
I find it amazing to have the chance to see the ships that are the best preserved Viking ships known and they were discovered in royal burial mounds around Oslo. According to legends, they were used to carry the dead to Valhalla together with their treasures that are now exhibited at the museum.
There are also temporary exhibitions or other cultural events at the museum as it is a fantastic place for folk concerts. The museum is located in the Bigdøy area, together with other museums full of Norwegian history and a visit in that area is a perfect way to spend a lazy day and learn about the old ages. It is also one of the places where I like to spend time in the souvenir shop since they tend to have cool (but expensive) stuff there.