Soak in an outdoor bath high above the sea
These are rotenburo, open-air hot-spring baths, set on hillsides and clifftops so that the sea fills your whole view as you soak. All of them take day visitors, so you can spend a slow half day bathing, resting and bathing again. They are far apart, two on the Izu coast southwest of Tokyo and one in Oita on the island of Kyushu, so pick the one that fits your route.
This day-use bath house stands on the coast at Izu Kogen, and its open-air bath sits high enough that the Pacific and the sky seem to run on without a break. After soaking you can stretch out in a lounge that also looks over the water. Izu-Kogen Station is about two hours from Tokyo by limited express, and the baths are a short taxi ride from there.
The highlight here is a tachi-yu, a standing bath deep enough to soak in on your feet, about 25 m long and facing Sagami Bay. Standing at its edge, the water seems to spill straight into the sea and the horizon. It is in Atami, a hot-spring town about 40 minutes from Tokyo by shinkansen, and a short bus or taxi ride from Atami Station.
From a hillside above Oita city, the open-air bath looks out across the whole of Beppu Bay. The water is a mol spring, or moor spring, which takes its soft, smooth feel from ancient plant matter underground, and it flows straight into the bath without being reused. It is about five minutes by taxi from Nishi-Oita Station, and staying into the evening lets you watch the light fade over the bay.