“Nice free park, good for relaxing or excerise”

Yoyogi Park is a good choice, and probably the most visited park in Tokyo. Due to its close proximity to Meiji Shrine, Harajuku, and Shibuya, it acts as a good central location and often features events in the nearby art space. A visit to Yoyogi Park on a Sunday morning, and you will see numerous (hundreds) of people jogging, dancing, or doing aerobics; and on a Sunday afternoon, a festival will most likely be taking place nearby. A good park, good size, very good.