Müggelsee

The most peaceful and relaxing place in Berlin (of course, closely followed by my favourite, Treptower Park): the Müggelsee.
How to get there? Take the S 3 to Friedrichshagen, walk down the beautiful Bölschestraße (the "Ku'damm" of Eastern Berlin) and you will reach the lake in 5-10 minutes.

There is a great webpage (German only, unfortunately) with information on trails and tours and even an own travel guide app for smart phones.

There is hardly a place in an urban area I know where the eye can wander that far without being disturbed by some construction or other at the horizon.

Fun fact: earlier this year a movie called "Haialarm am Müggelsee" (Müggelsee shark alarm), sort of an (East) German (anti) road movie, turned the area into somewhat of a famous local screen beauty.