That the district Weissensee is named after a lake is quite self-explaining, but that it is an actually pretty nice lake to swim in, I didn’t know for many years. It is one of the only lakes that is located rather close to the “center” (which means Mitte, although I know there is no Berlin center) and reachable by bike (about six km from Rosenthaler Platz) and therefore just perfect if you don’t want to go out to Brandenburg or to a public pool. Still, many Berliners think alike and so you’ll meet many of them there.
Nevertheless, it’s a lake! To swim in! (Almost) in the middle of Berlin! And it has a fountain in it! And also a small café and a Strandbad, means a small sandy “beach” to pay entry for. Around the lake are enough small spots in between trees to leave your bike and your belongings and jump into the (harmless) water. Unless you dislike water plants, dragonflies, small fish or muddy grounds, because, as I said, it’s a natural lake.