Swimming in German lakes

This weekend my colleagues invited me to go by car to a nearby lake for swimming and sun bathing, so I had to go look up which lakes around Aachen are good for swimming and here are some sites I came across (check out the Eifel a bit south of Aachen which is just a Natural Paradise with Rursee swimming, sailing and Monschau Alt Stadt bordering one of the lakes!):

Aachen sites
http://www.aachen-emotion.com/en-students/snapshots/treasure-hunters-in-natures-paradise
http://www.aachen.de/EN/kf/freizeit_en/index.html

Eifel NaturPark area
http://www.jaegersruh-eifel.de/Umgebung.htm
http://www.naturpark-hohesvenn-eifel.de/
http://ferienhaus-eifeltor.de/ausfluege/wandern.html
http://www.rursee.de/touristik/rursee.php

Monschau
http://www.fewo-renate.de/index.php?op=umgebung

Interestingly, the secretary of the institute here also asked me how i like the new facilities and we discussed that even though I am going back to the US in a few weeks, the next time (:-)) I will know where to start my time in Germany.
I am convinced I will be coming back here again soon:) Perhaps I will have to decide after visiting AUSTRIA!