Koganei Koen (koen stands for "park") is the second-largest park in the Tokyo metropolitan area and is located about 25 minutes from Shinjuku by train. The closest station is called Higashi-koganei and from there it is about a 20-minute walk. The park has multiple entries and exits, so you might want to keep checking you locations on the various maps or route boards. There is so much tranquility there, so if you are looking for a quiet time from your busy life in Tokyo, this is the place you should be!
Find out more about Koganei Park.