There are quite a few museums and art galleries, too, but I haven't been to any 😱, so you'll just have to trust the tourist info or hear what others here have to say.
I also know that Holmenkollen is the most visited tourist attraction in Oslo. I've been there once at a restaurant, but I haven't seen what else it has to offer. It's a Nordic skiing arena, it has a nice view of Oslo and I think there are some museums there.
You might want to go and have a read of tripadvisor.com and put in Norway.
Hotel, restaurant and attraction reviews will spring up from hundreds of people across the world. Their forum is pretty good too for specific travel questions - they have one for most cities (some obviously busier than others)
They have some pretty famous waterfalls there...you'd have to ask someone more knowledgeable
There's the Ibsen museum, (if you like 19th century playwrights that is)
I only went really for skiing, so that's all I've got really. sorry.