Hi Alice! 

I am in Norway right now, so I can share a bit of my experience if you want. 

I'm sure all the fjords are beautiful, I don't think you can really go wrong here. Personnally, I went to see Hardangerfjord, which you can easily reach from Bergen. On all the fjords, a lot of companies run day-cruises; however, those are generally quite expensive, so what I did instead was take a bus from Bergen to Norheimsund (bus 925 if I remember correctly) then the ferry on Hardangerfjord. Since this ferry is technically "public transport", it's a little bit cheaper than cruises, even though it's still mostly for tourists. You can choose to stop in any of the villages around the fjord (choose when you book online) and then take the return trip in the afternoon. I had originally chosen Eidfjord, but changed my mind at the last minute and opted for Ulvik instead. It was beautiful and very peaceful (and gave me a little bit more time "on land" than Eidfjord). I'm sure all the stops are a good choice though.

 The train ride from Oslo to Bergen is really as beautiful as people say. Apart from this, I also went to the centre of the country, first Lillehammer then Dombås, and I will be going to Hjerkinn and Trondheim next. 

I would recommend downloading the Entur app : it has all trains and bus schedules. 

If you have any other questions, I would be happy to answer :)