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.
A very good reason to travel West: the C/O, next to the formerly infamous Berliner Zoo tube station, is an awesome museum dedicated to photography and visual media. Previous exhibitions include Larry Clark, Martin Parr, Anton Corbijn, Annie Leibovitz and Stephen Shore.
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.