It really depends what you like. Regarding food, I have never been to restaurants like Maemo because it's really expensive (it has michelin stars so...). If you want some good "junk food", try illegal burger, my favourite burger
You have classic stuff to visit like Holmenkollen (ski jump on the hill outside of Oslo), Vigelandsparken (naked statues crying and fighting), the Munch museum, the Opera (it's shaped like an iceberg), Karl Johan street with the king's castle at the end.
But then you also have the more new and "hip" stuff too:
- Aker brygge with the Astrup Fearnley museum at the end (they have a Murakami exhibition right now).
- the opera could also go here too, it's pretty new, along with the barcode buildings next to it (no need to visit them, it's mostly offices, but they look cool from a distance)
- Following Akerselva (Aker river) and go to places like the Vulkan area and Grünerløkka (Oslo's hipster town with "vintage" stores and coffee)
- the Bar Himkok has been on the list of best bars in the world. You have to be 25 to get in and it's a bit hidden. They have a thing called taptails, cocktails from tap.
If you want to party, here's how it works in Norway/Oslo. It's expensive to drink in clubs, so we have pre-parties to around 23-24.00 before we go out. The clubs and bars close at 3.00. There's a time limit to buy beers in stores to 20.00 on weekdays and to 18.00 on Saturdays. You have to go to a Vinmonopol to get stuff stronger than beer, they are open to 18.00 on weekdays and 15.00 on Saturdays. If you arrive by plane, buy something in the tax free, it's much cheaper. Clubbing in Oslo is posh or wannabe posh people on the west side, hipsters at Grünerløkka and a more normal mix of people around Youngstorget. There's actually a bar next to Youngstorget called Oslo Camping where you can play mini golf inside. Never been there though.