The Yukemuri Observation Platform looks over Kannawa, Beppu's most atmospheric hot-spring district. From here you can see columns of steam rising from bathhouses, vents, kitchens, and ryokan rooftops, turning the neighbourhood into a living geothermal map.
It is a short stop rather than a major excursion, but it explains Beppu better than another bathhouse facade. Go in cool weather or early morning if you can, when the steam hangs thickly and the town's onsen machinery feels wonderfully visible.