It's been a long time since I was in Beppu, but it's very commercialized, even by Japanese standards. It is famous for geysers and bubbling hot springs, but they are all privately owned, I had to buy a series of tickets to go see them. I recall there being one that made up of boiling mud and another that had red water.

If you have only a limited time, Kagoshima, while a pleasant enough city, is not a priority.