Theres a mixed onsen in the Hakone area that is very popular with English speaking tourists. They have an indoor and outdoor mixed bath area. Its quite low key compared to the big resort onsens, its a family owned joint. But we had a great time there. And if you have a big tour group there you can all get naked together if you feel the need. Unfortunately there wont be any hot local Japanese girls in there though.
http://hakone.syuriken.jp/hakone/
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