Heiwajima Natural Hot Spring

In a savvy move, this 24-hour bathhouse offers a free shuttle bus service to nearby Haneda Airport every morning – perfect if you've got an early flight and would rather wait in a hottub than the departure lobby. The baths at Heiwajima Natural Hot Spring are designed to treat a range of ailments, from insomnia to lower back pain, and – as the name suggests – all of them use bona fide onsen water, drawn from 2km underground. If you're looking to get some sleep, there's also a relaxation room filled with row upon row of reclining chairs. Note that there's a ¥1,900 surcharge between 8pm and 10am, and the overall price rises by ¥200 for each hour that you stay over seven hours.