Tryvann is a nice little area within Nordmarka (another afar.com highlight), that has everything you can ask for in a forest.
Tryvannshøgda, a large hill, overlooks the three small lakes: Skomakertjern, Lille Tryvann, and Store Tryvann.
The television mast Tryvannstårnet on top of Tryvannshøgda is open to visitors, and on a clear day you can see all the way to Sweden from it.
Tryvannstua, a small cabin that serves snacks and refreshments to hikers, is open year-round.
If you're into skiing, Oslo Vinterpark has 18 slopes and is Norway's 4th largest ski park.