On our first day, departing from Haneda airport, we arrived at Oita airport and then travelled to Beppu. Talk about geothermally active... you can't help but notice the vapour trails climbing out of the Beppu landscape. Lonely Planet tells me "over 100 million litres a day of near-boiling water gush out of more than 3000 springs."
We spent our time wandering around the nine Jigoku, natural hot springs on show rather than for bathing. You can see Aki in one of Jigoku areas above. While they are touristy, and you may wonder why elephants, crocodiles and other strange denizens are being subjected here to sub-zoo conditions, the springs themselves are interesting to see.
And of course, we had to end the day in a natural onsen. We had intended to try out a sand bath as well, but time was against us.