A few cheap things to do.... Catch a train to Skådalen or the next stop up the mountain and ride a sled or walk back down the mountain back into town. Obviously you will need SNOW to do this on a sled :-) Visit Holmenkollen ski museum and ski jump. Go into a supermarket and buy one of their disposable/portable barbecue trays (all self contained) and cook a meal outside in the snow, or in Vigeland park. Go to the Viking museum at Bygdøy. Also walk around there for the day. Enjoy!!