Trollvann is one of the most beautiful lakes in Oslo. For me, it is the most beautiful one! One of the best places to relax and enjoy nature. Trollvann is easily accessible by bus (always check bus departures and timetables at ruter.no!) or a 40-minute hike from Stig bus station. One does not need to be an experienced hiker (although the hike does include a steep slope of a few meters), but then again, you can always reach the lake by bus or car through another route.
You can visit Trollvann all year round. It is allowed to swim and fish there, as well as grill and have a picnic. Free camping is also allowed in Oslo and that is definitely a plus! Of course, the lake is frozen during the winter, but it is still popular among the locals because of the ski facilities. There is also a little cafe near the lake (Trollvannstua) so you can always enjoy freshly baked waffles. I recommend visiting the lake at least once every season, that is how you can appreciate it in all its beauty. And in the summer and autumn especially, it is like being in heaven!