Hello Serulane,
I just returned from Norway and did a similar trip to the Geirangerfjord in a Nutshell, but did it on my own, all by public transportation. I'm not sure how much time you have, but here is what I did in 12 days:
I started out with 3 nights in Oslo, then did the Norway-in-a-Nutshell trip by train, boat & bus to Bergen in 1-day. Spending 3 nights in Bergen (2 full days), then flew to Ålesund and spent 2 nights, before taking a boat cruise through the Geirangerfjord to the town of Geiranger, where I spent another 2 nights. Geiranger is very small and if there are cruise ships in town (which there are frequently), it's a bit crazy. The day we arrived, it was gray and drizzly and a large cruise ship had just arrived, dumping 5,000 people in the tiny town. We began to rethink our choice of staying there, but the next day we lucked out. For our full day in Geiranger, the clouds parted and it was a beautiful day. Best of all, the large cruise ship left early and there were no other cruise ships!
We hired a taxi (arranged through our hotel) to drive us up to several view points for fantastic views above the fjord: Flydalsjuvet, Dalsnibba and Eagle Bend (Ørnesvingen). It was expensive (around $300), but our only option, as the buses that generally go up to the viewpoints only run when there are cruise ships in town. Split among 4 of us, it well worth the cost,
The next day we took the bus from Geiranger via the recently opened Trollstigen road to Åndalsnes, which was amazing. We stayed one night in Åndalsnes, taking the gondola to the top of Ramsdalen the afternoon we arrived for fantastic views. There are quite a few hikes in the area, so if you want to do some hiking, it would be worth spending another night in Åndalsnes.
Our last day we took the train from Åndalness to Dombås and on to Oslo airport, where we spent our last night before flying home.
I recommend booking hotels and all transportation ahead (most trains and buses can be booked 2 months beforehand). The Norway in-a-Nutshell trip can be booked through Fjord Tours - www.fjordtours.com/en
Bergen-Ålesund flight via Wideroe Airlines - www.wideroe.no/en
Ålesund-Geiranger boat via Geiranger Fjord Service - www.geirangerfjord.no/en
The Geiranger-Åndalsnes bus via Trollstigen and the Åndalsnes-Dombås-Oslo trains were purchased through www.vy.no/en
I used Rick's Scandinavia guidebook for hotel recommendations in Oslo, Bergen, Ålesund and Geiranger and www.booking.com for Åndalsnes and Oslo airport.
Hope that helps!