Norway

Oslo is great for swimming. Sørenga by the Munch museum is really nice, and a swim next to the Opera can also make for a really nice experience. Bygdøy has great swimming spots a bit outside of the city, and this is also where you will find the viking ship museum. For a swim in the river, there's a great spot around Nydalen. The lakes (like Sognsvann) are a bit more outside the city and can be colder, but going up there and sleeping in a hammock by the water can be a great way to combine a nice experience with saving money on hotels. If it's cold, check out Oslo Badstueforening for really affordable floating saunas.

I guess in general what to do depends a lot on your interests. If you just want to walk around in the city a bit, Grünerløkka is known to be one of the nicer neighbourhoods.

I left a while ago, and since then both the Munch museum and the National Gallery have moved locations. People say both are pretty great, so if you're into art they're probably worth checking out. Astrup Fernley is more geared towards contemporary art, which is good if you enjoy the whole "is this art" debate and all that jazz.

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.

Best meal I ever ate: https://maps.app.goo.gl/bkbS5vMA8b1inCSD7

If you want something much less fancy, Bislet Kebab is my usual when I can't be arsed leaving my hotel.

If you're I to edible souvenirs, try Dundersalt. Can be found in the candy aisle of any grocery store.

Dried lutefisk is just dried fish (Tørrfisk). Lutefisk is what you get when you add lye to tørrfisk, before it is rinsed for a few days. In other words, you can get lutefisk at many fish oriented restaurants. Tørrfisk can be bought at fish markets, and is a better choice if you want to travel with it. It's somewhat seasonal in terms of tradition, but you can probably find it all year.