My favorite lake to visit during the warmest months. It's located in the southwestern part of Berlin and it's reacheable by public transportation, just around 40min from the city center.
Haus der Materialisierung is an association of various local initiatives and institutions that research, practice, and work on the topics of sustainable management and climate-friendly use of resources.
...and it works!
I never heard about Havel until some friends and me checked out the google earth for a trip at the weekend. We went there in the german winter, oh yes...in the winter. I loved this place because is a small city island with a lake (which was frozen at that time) and with lovely houses and an old mill. I have never been there in the summer but i can imagine how wonderful would be.