Norway in a Nutshell

I did it in one day, because I didn't want to extend my trip any longer, and I wanted to hedge my bets in case we had a rainy day for the fjord cruise on the tour. Turned out the tour day was sunny and the NiN day was very rainy. It depends on what you want to see/do along the route if you want to do the route over more days.
I think the Flåmsbana train was the best part.

We have done the NIN twice. The first time we went from Oslo to Bergen all in one day. It was really long! Most recently, we took about a week. We went from Oslo to Stavanger to Bergen back to Oslo. This was much more enjoyable. IMO, anything in-between this would be good. One day is just too long of a day, IMO, so I would at least try to overnight somewhere in between Oslo and Bergen.

In 2023 we did a DIY Norway in a Nutshell between Bergen and Oslo. We overnighted in Flåm. This allowed us to take the 2 hour tour to Stegastein Viewpoint. You could always add another day or so if there are specific activities you would like to do along the route such as hiking, kayaking...

Here's a link to
FjordTours to get an idea of the various activities in Flåm, Gudvangen and Voss

I also did a DIY nutshell and overnighted in Flam. Definitely good to break up the trip it would have been a very long day. Read up on the details for buying your own various tickets, each varies slightly and September may still yet be busy. Or you could buy a package deal and save the hassle, I think some companies allow an overnight stop.

Since the tour visits Oslo check to see if there are other things you want to do before heading home. I stayed at a hotel near the train station that has a quick connection to the airport. Not the nicest area but certainly convenient for the incoming NIN train and the airport train.

I am doing Nutshell in June and will begin in Oslo (arriving from ferry from Copenhagen) . I will spend a night in Aurland, two nights in Flam - just want to soak it in while I'm there and also plan to take the Fjordsafari to Undredal one of the days in Flam. Then on the ferry to Gudvangen then bus/train to Bergen. I will have two nights in Bergen at that point.

We went from Oslo to Bergen and spent 2 nights in Flam. I'm glad we did 2 nights because the weather was horrible the day we arrived in Flam. It was snowing and sleeting on the Flamsbana and we couldn't see a thing on the train ride to Flam. So it was nice to have another day. This was in June.

my party of four did the NIN, Bergen to Oslo in July 2025. Our trip went as follows:
Ferry Bergen to Balestrand
2 nights in Balestrand , included a morning trip to see the stave church in Vik. Also enjoyed the interesting Museum of Tourism.
Morning Ferry Balestrand to Flam, then continued the fjord scenic cruise to Gudvangen, then took a bus back to Flam.
Stayed one night in the hostel in Flam.
Morning Flamsbana Train, Flam to Mydral
Train Mydral to Oslo, arriving in early evening

we enjoyed the fjords, without rush.