All the good points of a spring onsen experience, but you won’t have to share.
The Ryokan with a variety of hot spring baths can have a kashikiri bath. This is a bathroom that can be used privately by locking the door behind you. These can be used alone or as a family or as a couple. Higher-category ryokan may also have rooms with private en-suite thermal baths, either indoor or outdoor, perfect for couples.
However, take an onsen bath, the end goal is the same: achieve a state of yudedako or “boiled octopus” also known as the feeling of happiness and relaxation that you get after total immersion in hot bath water. Enjoy!