I found this peaceful place by chance, so sorry that I can´t tell you where it is exactly. But it´s in Düsseldorf (Germany) and it´s called the Japanese Garden.
Why I like it?
Although it was near the street it was a really nice and quiet place. I liked how the trees were cut and the cute details with the stones and the lanterns. I really felt like I want to sit down in the grass and meditate. Maybe it´s a cliché to think something like that but every Japanese garden I have seen so far, gives me this feeling of peace and silence. I think some of you are feeling the same.
P.S.: And yes it was free (when I was there).