KAI Aso, Kyushu, Japan: A boutique resort in Aso-Kuju National Park

It's not every night you get to dine at a fancy restaurant – in your pyjamas. But that's exactly what I'm doing in the dining room of Hoshino Resorts KAI Aso, a boutique hot-spring resort in the volcanic heart of Kyushu, the southernmost of Japan's four main islands.

Actually, they're not technically my pyjamas. Like other diners around me, I'm dressed in KAI Aso's version of "onsen wear": mauve cotton PJs, a soft grey linen yukata, grey split-toe socks to wear with a pair of wooden thongs, and a short quilted coat (we're 1000 metres above sea level so nights can get chilly).

Clip-clopping back to my room after dinner that first night, I feel quite the geisha.

When I say "room", I mean freestanding villa. There are just 12 of them at KAI Aso, each with a Western-style bedroom (real beds, no futons), a tatami room for relaxing, a lounge room with skylights and timber floors, and sliding glass doors opening onto the surrounding forest.

It's outside that I find KAI Aso's real drawcard, the reason people come to this boutique resort within Aso-Kuju National Park. It's my own private bath built into a timber deck and filled continuously by 40-degree spring water.

Throughout my two-night stay, every chance I get, this is where you'll find me: chin-deep in clear, volcanically heated water, preferably with the late afternoon sunshine twinkling through the trees and a bit of birdsong. It's "forest bathing" taken to a whole new level.

Lest I par-boil myself, a 10-minute "hour glass" is provided. That's the ideal length of time for a soak, according to the "art of bathing" guidebook on my coffee table, which includes information about the mineral content of the water, how volcanoes create hot springs, deep breathing techniques, even in-bath stretches.

Like Hoshino Resorts' 14 other KAI hot spring resorts around the country, helping guests connect with their surroundings is part of the KAI Aso ethos. .

So on my first morning I try "mountain-air breathing" at 7am, a sort of standing yoga class outside the main building, with misty mountain views. This is followed by breakfast in the company of five mountains that resemble a sleeping Buddha, including Kyushu's highest peak, a 1791-metre active volcano called Nakadake. There are free springwater- and sake-tasting sessions throughout the day, too.