As we are living in Norway and only 50 km from Oslo we think Oslo is a nice place. Scapel has described the best of Oslo that you can visit in one day. Remember, Oslo is expensive.
Stockholm and Gamla Stan is very nice, lived there too :-) Wasa museum is nice to. Slightly cheaper.
I think Scapel has made a good description of both Oslo and Stockholm. Ther people is very helpful.
Have a great trip.