Lutefisk in dry form is called tørrfisk, and you can get it year round.

I’m not from the area, but I would say if you want to learn Norwegian history, take the ferry to the Bygdøy museums and walk around. Also, Frogner park, which isn’t too far from the castle. The national art museum is also worth a visit. There’s a ton to do in Oslo. They’re just my favorites.

Stop by a tourist information kiosk she pick up their guide. They may have discounts, and ask about a public transportation pass. One time I was there we used that to hop on and off buses, subway, etc for one price. Pretty good deal.

Expect to spend more money than you think, but don’t bother with cash. I have not needed cash when I visited Norway for more than a decade at least. Even public toilets take cards these days.