Oita

They are half an hour apart and they attract almost opposite trips, which is why so many itineraries pick the wrong one. Beppu is a working town of some tens of thousands built on top of an enormous thermal field: steam rises out of drains and back gardens, the bathhouses range from polished hotel facilities to tiny municipal rooms that residents use on the way home, and the atmosphere is unpretentious and slightly industrial.

Yufuin, inland under a twin-peaked mountain, is the curated one — a walkable main street of galleries, boutiques and dessert shops leading to a small lake, with the serious bathing hidden inside expensive ryokan rather than open to passing visitors. If you want to soak cheaply and often, base yourself in Beppu. If you want one carefully staged night with a private bath and a kaiseki dinner, book Yufuin and accept that the daytime street belongs to coach parties. Doing one night in each is a legitimate answer and the one most people should take.

The Sonic limited express runs from Hakata to Beppu and Oita in about two hours; the Yufuin no Mori tourist express takes a slower inland route to Yufuin and needs reserving well ahead. Oita Airport is on the Kunisaki peninsula, about 45 minutes from Beppu by bus, and is roughly an hour and a half from Haneda. Beppu's hells are spread over two clusters linked by local bus, and the town itself is walkable in parts. Yufuin's centre is small enough to cover on foot from the station. Kunisaki's temples and stone carvings are scattered across back roads with almost no bus service — take a car.

The prefecture has more hot-spring sources and discharges more thermal water than anywhere else in Japan, and the everyday consequence is that steam gets used rather than admired. In the Kannawa district of Beppu there are public steamers where you buy a basket of eggs, sweetcorn and vegetables and cook them in a vent in the ground, a domestic technique that predates it becoming something visitors queue for. Households nearby have historically drawn the same heat for cooking and warmth.

The same resource runs at industrial scale further inland, where geothermal stations generate power from the field, and at neighbourhood scale in the small unstaffed bathhouses that dot the older streets and charge a coin into an honesty box. Knowing this reframes the famous ones: the coloured "hells" are viewing ponds, not baths, and skipping them entirely in favour of an ordinary municipal bath is a defensible plan rather than a missed sight.

Two days, and Oita is the most concentrated hot-spring prefecture in Japan — it produces more thermal water than anywhere else in the country, and Beppu alone has eight distinct spring areas within the town.

Beppu and Yufuin are the two names, about an hour apart. Beppu is a working hot-spring city with steam rising from the streets, sand baths, mud baths and a set of "hells" that are for looking at rather than bathing in. Yufuin is smaller, prettier and more curated, in a valley below a twin-peaked mountain, and is a day trip or a night.

Day three, if you have it, goes to the Kunisaki peninsula, where a distinctive Buddhist tradition left stone figures and clifftop temples across a set of radiating valleys, and where almost nobody goes. That needs a car.

Onsen towns work best when the air is cold, so November to February is the natural window, and Oita's winters are mild with January highs near 11°C and little snow at sea level. The Kuju highlands inland are colder and do get snow, with alpine flowers in June and colour in late October. Summers are hot and humid above 32°C — Beppu in August is a lot of steam in a lot of heat. The rainy season in June is heavy. Beppu's Onsen Matsuri falls in early April, and the Kunisaki stone-Buddha routes are best walked in spring or autumn.

Toriten is chicken in a light tempura batter served with ponzu and mustard, an Oita invention found in ordinary restaurants across the prefecture rather than specialist ones. Seki mackerel and seki horse mackerel are line-caught in the fast tidal water of the Bungo Channel off Saganoseki and are graded and priced far above the ordinary fish. Dango-jiru is a flat wheat-dumpling soup, farmhouse food. Bungo beef is raised inland, and the steam from the hot springs is used to cook vegetables and eggs in public steamers at Beppu's Kannawa district.

Trains for Beppu and Yufuin — both are on the line from Fukuoka, roughly two hours and two and a half respectively, and Beppu's buses reach the spring areas.

A car for the Kunisaki peninsula, which is a set of valleys radiating from a central mountain with temples and stone carvings scattered through them, and for the inland roads toward Aso. This is the part of Oita worth returning for and it has essentially no useful public transport. Note that Beppu is steep — the good baths are up the hillside, and the buses are the practical way to reach them rather than walking.

Nothing seasonally rules out a hot-spring prefecture; winter is arguably the best time to be here. The constraint is booking rather than weather: Yufuin is small, popular with domestic visitors, and its inns fill at weekends and through the autumn colour season.

August is hot and humid and sitting in 42-degree water is a specific taste in it, though the sand baths on the beach are pleasant year-round. The rainy season in June brings heavy rain to central Kyushu and the mountain roads toward Aso are affected. Beppu's steam is at its most photogenic on cold mornings, which is a small argument for winter.

A first visit is Beppu, and the advice is to bathe rather than sightsee. The "hells" — coloured, boiling, spectacular pools — are a ninety-minute circuit and are not for bathing; the actual value of the town is that you can spend a day moving between an outdoor bath on the hillside, a sand bath where you are buried on the beach, and a steam bath in a wooden box, for a few hundred yen each.

A second visit is the Kunisaki peninsula. Rock-cut buddhas, temples at the top of chained scrambles, stone guardians in the forest and a tradition that mixed Buddhism and mountain worship in a way that happened nowhere else. It needs a car and a day and it is one of the least-visited significant places in Japan.