Get the water boat across to visit the Viking Ships Museum and the Norwegian Folk Museum (a collection of old houses). Both very well worth a visit, but especially the ship museum.
Geoff
Just what I was going to say, both of these are superb and well worth a visit, the Kon Tiki museum is there too, too much to do in a day. The Viking boat museum is just the most amazing place, the ships, the building, awe inspiring. In Oslo itself the museum is good with a huge collection of Viking swords etc. I am told the Opera house is a stunning building to see but was built since I was last there. If you fancied a day out and are into Viking ships, axes etc you could visit the build of the new
Oseberg ship a short way south at Tonsberg.