What are some "must do" experiences when visiting Oslo and Arendal?

In Oslo for a few days, then passing through Arendal for a day.

Anything that I...

• Must see?
• Must do?
• Must eat?

Any recommended souvenirs to bring back to the land of sh.itjust.works? (I've already got some ammonium chloride licorice)

Would I be able to find dried lutefisk at this time of year, or is it seasonal?