The Wassermannsee rounds off the microcosm "Vogelsanger Alps" perfectly, because the mountains naturally also have a decent lake. Not as easy to see in summer as in winter due to the vegetation, but definitely worth a look.
Source Wikipedia:
Wassermannsee, also known as Wassermann-See, is a gravel pit in the Vogelsang district of Cologne.
The lake was created in the 1920s. The name-giving company Wassermann mined gravel there that an old arm of the Rhine washed up millions of years ago. After the end of the gravel mining, the area grew undisturbed and over decades. The access to the lake and its partly very steep embankment are largely closed. It can be used for fishing. The bank was reinforced with tall perennials and reed beds. The lake is almost 300 meters long and almost 100 meters wide.
The lake is part of the Vogelsanger Triotop, a three-part area consisting of the nature reserve with the lake and the near-natural meadow landscape Wassermannpark, the business park and a natural area, an orchard meadow.