Hokkaido’s Fukiage Onsen and Aomori’s Hiba Sennin Onsen are top-tier natural hot springs where mixed bathing is possible. They’re far away, but worth the trip. Aomori also has several other famous hot springs.

I think it depends on the woman’s perspective and the nature of the relationship. Asking other people about it is pretty tactless.

It really depends on how the woman feels about it. If she likes hot springs (or traveling) and is happy to go on a long date, she’ll probably enjoy it. But if she’s not interested, no destination or itinerary will work. Rather than the location itself, she may appreciate being taken to a higher-end ryokan—somewhere beyond what she could normally afford. Since it would involve staying overnight, though, I think you’d probably need to offer a higher allowance than usual. If I were going to a hot spring with my partner, I’d choose a room with a private open-air bath, since I’d be concerned about being seen. First, check how she feels about it, and try to incorporate her preferences into the plan. Incidentally, my partner says, “Forget hot springs—I wanna go to Okinawa!”

If you’re traveling on the Tohoku-Hokkaido Shinkansen, choosing GranClass rather than Green Car will definitely make you more appealing. I’d give it 85 points—though I may be grading generously. I hope your trip comes together. You may already know this, but Kirishima Onsen in Kyushu also has an open-air mixed-gender bath with a wonderfully—or perhaps overly—rustic atmosphere.