Agree with the advice given so far. May I suggest - especially if the weather is nice- from the Stn a walk to the New Opera House - then back across the bridge and towards Akershus Fortress (I found the Resistance Museum to be fascinating) From there a stroll to Oslo City Hall / Nobel Peace Centre Museum and then up to the Royal Park, where you can walk back along Karl Johans Gate to the Central Stn along a beautiful central park. I'm sorry I missed the Viking Ships and will check them out next time.