Many ryokan onsen will pick you up from a nearby train station with advance reservation First trip to Japan - suggestion for hot spring vacation ?

A few of the onsen ryokans in Kusatsu run complimentary or inexpensive buses to Karuizawa ~ 1.1 hr by shinkansen from Tokyo station. Ryokan shuttles are usually timed for afternoon arrival at the ryokan and morning departure. There's also public buses from Karuizawa or Tokyo if you'd like to extend your time there. You can send your luggage between the cities via takkyubin while you visit an onsen en-route.