I was just there. Definitely do the Sognefjord.
This is what I did. Note I booked this all on the fly as I was leaving Ærøskøbing - which is a tiny island in Denmark (and f*cking insane, everyone should visit once in their life). We went from Ærøskøbing back up into Copenhagen and then flew into Bergen on the west coast of Norway, which is basically the gateway to the fjords. Gorgeous town. Make sure to drink lots of acquavit.
One morning took the early train from Bergen through Voss and into Merdal. Just through the Norwegian countryside, unbelievable beauty. And stellar bar car on the train. Goes without saying but the little bit of extra coin for first class always worth it on these kinds of trains.
Then take the Flåmsbana train north into Flåm. This has been frequently voted the most beautiful train ride in the world. It's old school, kind of a "murder on the orient express" vibe and goes all the way down to the base of the fjord. It's the steepest non-funicular train in the world. Absolutely un-f*cking real. Sit on the left side going downhill into Flåm, and the right side going back up.
In Flåm, you don't have many options but I recommend the Aegir Brewpub - it's like super viking style with crazy beers, wild locals and legit food and right on the water. Stay a night in that town on the water. But check nautical cruise schedules because I hear once in a blue moon a small cruise ship will stop here, so make sure to avoid that shit. We were there and we were the only people in the pub who spoke English, which is the f*cking best if you are a drinker (which you are). Even the workers end their shift and get wasted with everyone until the sun comes up. Or could stay in Balestrand.
The next morning do a tour on the fjord. Do one of the legit ones on the small rib boats where you get the full gear. The one we booked (literally the day before arriving) had like 6 peeps scheduled but they goofed and overbooked, so I talked them into a private one with just my wife and I and a local guide. Unreal. Just, unreal. He took us out for hours longer than was booked for, even stopped off at his tiny hometown on the fjord and we went into the local brewpub. He owns lands and the woman renter has goats, she pays him in goats milk.
Take the Flåmsbana railway back to Merdal, and then then normal train through Finse and Geilo and into Oslo. Another world famous train ride.
Oslo was OK, but we got bored there after a few days and one night drunkenly booked a flight up the next morning into the Arctic Circle and stayed at the super famous ICEHOTEL in Sweden which had been on my bucket list for ages. That sh*t was out of this world crazy fun. Just un-f*cking real. Another thing everyone should do. Staying in a room entirely carved out of ice is hard to put into words. Cold though.
Any questions just let me know.