If they're unpopular because they're unknown, nobody will be here to tell you, and if they're secret, nobody will want to tell you here.

But to give you an idea: It gets easier once you're a bit past the city limits and you avoid those that get recommended everywhere. Check Google Maps, Open Street Maps and Komoot. You'll find beaches and small green spots on the lakefront.

E.g. take the train to Königs Wusterhausen, Bestensee or Storkow and cycle south/east, there are countless large and small lakes. Keep going to Senftenberg or Cottbus and you'll find lots of artificial lakes that used to be coal mines, it's quite touristy but there just aren't enough tourists to fill the whole area. Or go to Michendorf near Potsdam, Lienewitzsee still isn't too busy, but you could also go to Seddiner See which has a lot of beaches that can't all be full, or keep going further south east. In the east there are lots of lakes behind Erkner and Strausberg. North of Bernau the lakes around Wandlitz are rather popular but you could go further east or north of Eberswalde. Another popular but huge region is around Fürstenberg. Keep in mind that the regional trains going north from where the S-Bahn ends will be packed with Ostsee tourists though.