What's the nearest open air onsen to Tokyo?

My friend is visiting Tokyo. He doesn't have a lot of money but he wants to go to an onsen outside. Is there one anywhere near Tokyo that's outside?
What's the nearest open air onsen to Tokyo?
?i would suggest he go to the "Great Edo Hot Springs Story" complex which is located in Tokyo's Odaiba waterfront area, which has over the past several years developed into a fashion and shopping hub. Arriving guests remove their shoes and, after paying the entry fee, are directed to a selection of kimonos and colorful sashes. After choosing one, and putting it on in a locker room, guests drift out into what looks like an 18th-century remnant of old Edo — as the samurai called Tokyo — and can either eat, do a little shopping or head for the baths.

Unlike the old Japanese custom, men and women don't go in together here.

Bathers first take a shower, in order to get clean before entering the communal bath water. There are then several bathing choices — indoors and outdoors, with bubbles or without, hot or very hot, shallow or deep.

I've been here a few times and enjoyed it although I really love the traditional hot springs in Nasu Kogen, Hakone. This is in Tokyo and easy to get to. Also your friend will enjoy everything Odaiba has to offer. Hope he has a great time in Japn ?
What's the nearest open air onsen to Tokyo?
I think the nearest one from Tokyo is at Hakone, which is a tourist destination about 2 hours west of Tokyo.
I think the nearest one from Tokyo is at Hakone, which is a tourist destination about 2 hours west of Tokyo.