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Müggelsee is probably my favorite one in Berlin. It's quite big, maybe one of the largest in town, it got a proper sandy beach (which you have to pay for, unfortunately) and the little hill that overlooks the lake, only makes the view more interesting. On the hill you've got a bunch of restaurants and cafes from which you can admire a beautiful panorama of Berlin.
The lake is located in a really picturesque neighborhood that looks more like an independent village rather than an area of the city, also because, in order to get there, you have to go through a big forest, which separates it from the urban jungle.
Getting there it's gonna be an easy task, thanks to the great invention of the S-bahn. From Alexanderplatz, without changing, it only takes one hour plus 10 minutes walking from the station.
Something that I truly love about this place is the fact that it's surrounded by pure Nature, which, of course, makes everything more pretty. Water is crystal clear, overall a very clean lake. Restaurants and bars, not too many though, can be found around Müggelsee. If you want to have more culinary options, then I would rather go into town, which only takes 5 minutes.
Definitely a place that needs your attention!