You can notice it right away, even from your first visit to the city. Berlin is a green city. The high presence of green places combined with small lakes, ponds, and the river, allows you to relax, be alone, and enjoy your favorites open-air activities. The city offers its citizens a wide choice of parks, forests, and gardens, more than 2.500 public parks, covering a surface of approximately 6,5000 hectares.
Considering the great offer is always difficult to make a choice and pick up a few representative places. The only way to do it is to suggest you from my personal experience. As I moved to the city, I felt lost and disoriented at first. It might happen when facing a new environment and everyday life. It is called culture shock. My remedy to this is to find tranquillity in nature, were far from car noises and the continuous flow of people, I can clear my mind and reflect in peace. The first place I found was Planterwald, part of the more famous Treptower park, and a few hundred meters far from my apartment. It is the second-larger park of Berlin where catching a sunbath, doing a trip on a boat, or strolling along the river on a sunny day are only a few activities you can enjoy. Planterwald is more suited if you are looking for a more forest-like feeling, not far from the city center where to enjoy loneliness and silence. Between the two, Insel der Jugend is the perfect place to sit along the Spree or enjoy cultural activities such as poetry slam, exhibitions, and music nights.
The 386-hectare Tempelhof Park is a beautiful open space in the district of Neukölln, entirely dedicated to public use. Here you can skate, run, ride a bike, rollerskates or a longboard, fly a kite, walk the dog, meditate, dance, do some gardening, and so on. Tempelhof Park is a former airport which after its closure in 2008 was reclaimed by the citizens of Berlin for public use. A 386-hectare open space that strikes your eyes. Quite close to Tempelhof there is Viktoria Park in Kreuzberg, a borough characterized by a 66-meter-high hill. On top, a small romantic park with the only waterfall you can find in Berlin, a vineyard, and a national memorial. Southern-east of Tempelhofer Feld the underrated Körnerpark offers a palace garden feeling, an orangery houses, and an art gallery for temporary exhibitions.
These are the places that I have enjoyed the most and feel like to suggest to you. But if you have the chance take a look at all the other sites the city has to offer: Mauer Park, Freizeitpark Marienfelde, Tiergarten, Südgelände in Schöneberg, Monbijou Park, Forst Grunewald, and Volkspark Friedrichshain to name a few.
If you can bring or rent your bike, it's always fun to get lost in nature.