Bring lots of money, and a CARDY, even when the sun was out, it was pretty chilly quite early in the evening.
I was only in Oslo for a few days, most of them ESC-related, so I didn't do a lot of sightseeing BUT... we took a ferry from the marina to a bunch of museums. The Viking Ship museum was literally a cross-shaped building with three ships, so it won't take very long. The culture museum was quite nice now I think about it, the indoor part was full of old-style clothing and bits and bobs, which was nice but there was a LOT, but there are huge grounds with old-style farmhouses and things, if it's a nice day and you've got nothing else planned, do it!
The Munch museum was surprisingly good too, I'm not really one for galleries generally but I really liked it. I did like the bits of Munch I knew already from my GCSE art though, so maybe you'll feel differently.