Beautiful both summer and winter. It take 30 minutes with the t- bane from Oslo central station. You can walk around the water, and if you are in a mood for a hike you can walk up a small mountain called "Vettakollen". Here you will have view over the whole city.
Nice hiking in Oslo. Different distance and walks. In wintertime you can go skiing and in the summertime you can go for a swim, have sun, have a bbq or go for a great walk in the woods.
Recreation area with great places for picnics, swimming, fishing, walking, running and cross-country skiing. The two-mile (3.2-kilometre) walking/running path around the lake is lit and wheelchair-friendly.
Want to escape the city? Take the train to this park on the outskirts of the city. (I promise its beautiful there). Get a taste of Norwegian nature.