The Kon Tiki museum and its story is something I am only vaguely familiar with, but have now learned a lot more about. It houses souvenirs and the ship from the Kon Tiki expedition. I learned a lot about the men and what they went through, but I can't say that it was the coolest thing I've ever seen.
Literally just across the street is the Fram museum, and that was significantly cooler. I still didn't know a lot about the Arctic expeditions beforehand, but exploring the ship and playing the different games (even if they were meant for someone half my age) was fun. Apparently this is Oslo's best museum, but I'm not sure if I agree. Lots of color and stuff, but lacked a variety for me. But still worth the visit if you have the chance.