I recommend the Hama-rikyu Gardens founded in the Edo period. The place is like an oasis in Tokyo. You can relax under the numerous trees in the garden and still see the surrounding skyscrapers. It's a mix of old and modern Tokyo!
How to get to this park?
7 minutes walk from Tsukijishijo station (close to Tsukiji fish market), Shiodome station, 12 minutes walk from Shimbashi station