As a norwegian I should know these cities pretty well, but we tend to loose focus on the highlights in our own country.
Here's a list of things I find interesting:
Walk the streets and feel the atmosphere of a cozy western norwegian town. I you were hikers and not so afraid of heights it is wonderful to climb the 480 something steps to the top of the town mountain Aksla.
Have not been there since I was a small child, but I will be going there in the end of may. really don't know what to focus on here, but would guess it's a nice place to look at some craft made by the laps and their culture.
The polar cathedral (ishavskatedralen)
Polaria: Polar museum with the story of the arctic and a seal aquarium
As you suggest the folk museum.
Suggest you find a good brochure and pick what interest you the most. A must do is the fish market and the Hansa area with the piers. Again the height issue: There is a steep tram line to a mountain called Floyen which would give you a terrific panorama over the city.
In Nordnes there is an aquarium and a nice little park.
Have a wonderful time in Norway. I hope the weather will be on your side.