Manza-onse is very nice place as hot spring.
However you should visit in summer because there is ski area in winter.
Of course you can visit in winter but there is much snow and you might be difficult to reach. Personally I prefer to visit in winter by my car.
I can recommend you to stay either hotel.
Manza-kogen hotel
Rotenburo as outside spa is great!!
Nisshinkan
You should take a bath when you visit Manza.