Hatonosu (鳩ノ巣渓谷) is an unassuming little unmanned stop near the end of the Ome Line, towards the famous outdoor haven surrounding Okutama Station and Okutama-Machi (奥多摩). The station is unremarkable but sweet and rustic, and around it there aren’t many immediate amenities: a nice toilet, a small restaurant and a closed down, showa general store.
However, a short walk from the station takes you to a riverside path leading to a little footbridge over some of the most dramatic and scenic gorges, cliffs and blue river water in the Tokyo region. This is Hatonosu Ravine, and an excellent place for deep water river swimming.
Take a picnic or a barbeque with you and set up for the day, riverside camping is also possible, or those in the mood (and with slightly more of a budget) can warm up in a beautiful and very large onsen complex hidden from the river bay by a pretty forest.