Hyotan Onsen is one of Beppu's better-known public bathhouses, with a long history and a range of bathing options including indoor baths, outdoor baths, sand baths, and family-use private rooms. It gives visitors a fuller onsen experience than simply viewing the Hells.
As with any Japanese bathhouse, wash before entering the baths and check tattoo and private-bath policies before you go. It is especially useful for travelers who want a proper soak but are not staying in a ryokan.