Koishikawa Korakuen is one of Tokyo's finest traditional gardens, laid out in the 17th century with ponds, wooded hills, stone bridges, and winding paths. It is beautiful year round, with plum blossom, cherry blossom, irises, and autumn colour each taking a turn.
The garden sits close to central Tokyo but feels carefully enclosed from the surrounding city. Give yourself time to wander rather than treating it as a quick photo stop.