Sports cabin with café in Grefsenåsen in the north of Oslo, where you can enjoy a cup of coffee, warm apple cake or a simple warm lunch/dinner.
Trollvannstua is reachable by car (almost to the door), or you can walk, ski or bicycle there.
In the summer you can swim in the lake Trollvann next to the cabin. In the winter you can go downhill skiing in Oslo Ski Centre; one of its slopes ends right outside Trollvannstua. You can also go cross-country skiing in the many prepared tracks in the area.