For several years I lived across the street from Tokyo’s ‘secret’ garden, Rikugien.
Basically, it was my home away from home as I visited it every day.
Rikugien Garden (六義園) is arguably central Tokyo’s best–It’s constantly used by the Foreign Ministry of Japan to give foreign VIPs a taste of traditional landscaping aesthetics. And for extra fun, Rikugien Garden has several small teahouses can be privately reserved quite reasonably.
Check out the City of Tokyo’s official website for the garden at: teien.tokyo-park.or.jp/en/rikugien