I like to visit Matsumoto Castle, Tokagushi and Kamikochi. Which is the best area I should stay. Best with an onsen.
The closest area to Togakushi is Nagano City, but it doesn’t have many hotels with natural onsen. The Dormy Inn Nagano does have onsen and is also close to the station.
In between Nagano City and Matsumoto City is Togura Kamiyamada Onsen, a popular hot spring resort during the economic boom. For an authentic experience, consider staying here at an inn such as Kamesei Ryokan.
Matsumoto City is the most convenient area for visiting the castle and Kamikochi. There are several hot spring areas around here. The closest to town is Asama Onsen (see link for accommodations); Utsukushigahara Onsen is on the hillside with beautiful views of the city (consider Hotel Shoho; and Shirahone Onsen is a secluded hot spring area outside of Kamikochi (consider Yumoto Saito Ryokan).
I hope that gives you an idea of where to stay around Matsumoto and Nagano cities. Let me know if you’d like any other information.