Though a bit far from many of Japan's big cities, the trip to Oita is well worth it if you're looking for some hot springs. Oita boasts the most onsens in Japan (over 4,500), including Beppu and Yufuin, the country's two largest hot springs areas.
When you're not bathing and relaxing, experience a bit of Japanese culture first-hand by visiting Usa Jingu Shrine and the unique Usuki Stone Buddhas. Hikers will enjoy the nice views from Mount Yufu and Mount Kuju, while children are in for quite an adventure at the African Safari Park and Umi-Tamago (literally "sea egg") aquarium.
Tourists can also participate in "green tourism" and "blue tourism," short-term homestays at farms and fishing villages respectively. Don't forget to have some tori-ten, or chicken tempura, while you're in Oita too.