Izumi Tenku no Yu is a 24-hour spa directly connected to Haneda Airport Terminal 3, featuring around a dozen hot springs and baths. Relax in the outdoor bath while watching planes come and go, and even catch a glimpse of Mt Fuji on clear days. There are also plenty of couches and reclining chairs, plus dry, steam and stone saunas.
It costs ¥4,800 for adults (¥4,000 between 1am to 5am) and ¥2,000 for primary school students and under. The admission fee includes a face towel, bath towel and access to the entire facility. Those staying at Hotel Villa Fontine receive a ¥2,000 discount (¥1,000 for children).
Do note that baths are not available between 10am to 1pm daily, and that hotel guests have priority entering Izumi Tenku no Yu if the facility reaches capacity.