Aoni Onsen is a fabulous place, and it will surely remain fondly in our memories forever. It's a very isolated onsen in the middle of the mountains by a river. There's also a waterfall you can watch while relaxing in one of the onsens. There is also a mixed gender onsen, which was great as my girlfriend and I could stay together. The food was flawless, and gave us a feel of genuine Japanese experience (we were the only foreigners there). Highly recommended!