If you are doing the NiN route, that gets you to some of the best views on and above the Sognefjord. I wouldn't feel the need to return for another day on the Sognefjord. It would be different if you had more time and wanted to do a great deal of hiking in the area, but in that case it would be better to build in the extra time along the NiN route rather than doubling back from Bergen.
The Sognefjord is a UNESCO World Heritage site; the Hardangerfjord is not. I don't doubt that the latter is very nice, but I don't think it will be a huge "wow" after you've seen part of the Sognefjord. However, if you're looking for another day of scenery at that point in your trip and can find a reasonable-cost way go do the Hardangerfjord, why not?
I don't know anything about the glacier hike, but there's a financial risk involved in prebooking outdoor activities like that in Norway. You could get weather that's far from ideal, which would be a lot more irritating if the activity was expensive.
You might check on activities available in Voss. It's known as an adventure-travel center, and it's on the rail line heading east from Bergen. There might be offerings in Voss that are less costly than the glacier hike.
There are also, supposedly, good hikes from the tops of the two lifts in Bergen. You can even hike from the top of the higher lift to the top of the lower one. For that matter, just walking the streets in Bergen can be quite a workout. The area right around the harbor is mostly flat, but go a few blocks inland and you'll quickly understand why motorized scooters are so popular there.