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.
You can grab a coffee or a juice right next door to the KINDL Center in the old brewing Hall at König Otto, complete with all the fascinating brewing equipment from its completion in 1930 it is a hall reminiscent of German Expressionism style… or as stated on their website. :)
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.