Lake Ringsjön is a hidden gem. It is a beautiful lake, completely surrounded by forest, and that has several high and low cliffs along its edge for swimming or to get a nice view. There is a path along the entire east side of the lake, but it is difficult to traverse to the north and west side.
At the southmost part of the lake is a hill with a small improvised campsite with a firepit and some flat-ish ground.
Along the east are several nice spots for swimming and at the top of the east side is an overgrown double track road that reaches the lake.
Note that the area just east of the trail is a local shooting range, so mind the warning signs on the way there, and expect to hear noises from that direction, especially during weekend afternoons, but only during daytime.
Also note that the road Trosavägen passes not too far from the lake, so expect to hear some traffic noise at times.