Location: Hama-Rikyu Detached Garden, Tokyo
Hama-Rikyu isn't a landscape garden designed to recreate famous nature scenes from around the world, but it is a beautiful, large garden that provides a quiet place to stroll and enjoy greenery. Tea can be had in the teahouse and snacks are available nearby as well. Plan to spend an hour at minimum for a stroll around the garden - you could easily spend two or three hours here if you have a seat and relax in the fields. Plus, you can watch the water taxis go by from the east side of the island.
The garden is about 10-15 minutes from Shinbashi Station (less from Shiodome Station), and admission is only 300 yen. It's open from 9-5. A free English audio tour is available at the entry gates, and while I haven't tried it I've heard it is very good.