These comments are based on pre-trip research. I haven't been to Norway yet.
I think one of the key day trips from Bergen is into the Handangerfjord. I believe there are both boat and bus options. But having spent 3 days in Balestrand, you may have done all the fjording you need to do.
Three places outside Oslo that sound interesting to me are:
Drobak, described as a delightful town on the Oslo fjord.
Frederikstad, a popular weekend getaway with an historic fort and attractive waterfront. Just over 1 hour by train from Oslo.
Holmenkollen Ski Jump and Museum, which is out beyond Vigeland Park. There appear to be nice walking possibilities in the area.
I've given myself 4 full days in both Oslo (not counting arrival day from the US) and Bergen. I'm confident that won't be too long if the weather cooperates and allows me to enjoy time spent outdoors. I expect to be short of time in Oslo because of the museums, but I'll be looping back through Oslo on my way to Sweden, so I may be able to insert an extra day or two in the capital at that point. It will depend on whether I can maintain some flexibility in my Swedish hotel and train bookings. Costs in Norway are such that I'm going to try to concentrate on being an efficient traveler rather than taking my usual, laid-back approach of "What do I feel like doing today?"