Helgerendammen is a nice-size lake just north of Oslo
Helgerendammen is roughly a 10km hike from civilization, however the hike is quite pleasant, covering a few hills and passing a large lake along the way. Much of the forest is spruce, and there are a couple of scenic spots. But of note, when wet out, best not to walk on the rocks without great care, as they become very slippery. The ground was super-saturated on this hike after weeks of wet weather, and it rained at least half the time, but other than the usual miseries associated with hiking in such cold and wet conditions, we very much enjoyed ourselves.