The Korketrekkeren is a famous tobogganing track at winter, where both adults and kids pick up or rent sledges and slide down for quite a distance on a track that follows an old Olympic bobsleigh run.
Its biggest advantage is that you just take the subway 1 to the last station, Frognerseteren and walk down to the renting cabin, Midstuen (they have a page in Norwegian though). And from there few more steps to the beginning of the track and you’re on your way down for about 1.5 km filled with speed and fun. Not to mention the amazing views over the city and the lovely nature dressed all in white. And once at the end of the track, you jump in the subway again and go back up.
Usually there are people selling warm food and drinks. But you can also stop at the cabin for that. One can drive up there and bring their own warm stuff in the car, but it’s way funnier to take the subway with everybody else.
It is possible to also ski around the area but I haven’t done it yet, so I can’t give many details at the moment. If interested, drop me a message and I can try to find some details.
Info about weather and a webcam can be found at the Norwegian sleigh association, same link as above. Videos can be found on youtube, so you can see how much fun it can be.