"I love red wine, it's my favorite thing!" These are perhaps not the words that a mother particularly likes to hear from her young daughter — especially when she is only 4 years old.

But this was precisely what my daughter told me when we recently paid a visit to one of Japan's less conventional attractions: an onsen (hot spring ) theme park called Yunessun Spa Resort.

Located in the mountains of Hakone, Kanagawa Prefecture, Yunessun is hot-spring nirvana — with a quirky twist. Not only is it home to an expansive Japanese-style hot-bath experience, it also has an array of unique offerings, in particular, a series of baths made with ingredients such as coffee, green tea, Japanese sake and, my daughter's favorite, red wine.