Iwakura Onsen
Iwakura Onsen is the only hot spring resort in Tokyo that is said to have started providing hot water over 1200 years ago.
According to legend, the origin of the name "Iwakura" was derived from the armor that Yamato Takeru placed in a storehouse (kura) made of rocks (iwa) when he bathed in the hot spring.
There are hot spring facilities that you can use for a few hours, and you can also enjoy fireflies in the summer as well as the autumn leaves.
There are sightseeing spots such as Shiofune Kannon-ji Temple, Fukiage Shobu-en (iris garden), and Anraku-ji Temple in the vicinity, so feel free to also visit those spots.