Near Waseda University in northwest Tokyo is a really nice garden called Okuma Garden. It’s just behind the main clocktower on the university campus.
To get there, take the Metro Tozai Line to Waseda Station, then head north (right) up the street. At the next major intersection, turn right (east) and you’ll be on a charming little side street. Follow the street to the end and you’ll see the clocktower. The garden is just to the southeast behind it.