You might consider an onsen in Hakone or Kawaguchi-ko. The deluxe onsen review thread pinned at the top of this forum has recommendations in both locations. Both recommendations are for FuFu properties. I don't know if the authors have any relationship with FuFu or the other properties that they reviewed. Needless to say, other deluxe properties exist. (Open the thread and do a word search for "hakone" or "kawaguchi").
Japan Luxury Ryokans - A Primer + Impressions

Ikyu is a good site to search for luxury onsen if you can cope with a japanese interface.