I would suggest you to visit Mt. Aso - It is the biggest active volcano in Japan and you can hike it up. It is between Kumamoto and Beppu city (2hr by JR train from Beppu / 1hr from Kumamoto).
If you were looking for relaxation after the hike I would suggest Beppu City -
the most famous onsen city in Japan (mud baths, steam baths, sand baths, onsens) and wild onsens in mountains above the city.
Beppu, Ōita - Wikipedia, the free encyclopedia
There is a Monkey Park between Beppu City with about 1000 wild Japanese makaks. The park is about 30 minutes by bus from Beppu.
Beppu Travel: Takasakiyama Monkey Park
HAPPY NEKO - Budget accommodation in Beppu, Japan