The serene island that is Japan's best-kept secret for adventurers

From an empty sun-kissed beach on Himeshima, the rugged mountains of the Kunisaki Peninsula ripple on a distant shore like the back of a dragon - a vicious one, at that. I know because this morning I was traipsing atop those ripples, hauling myself up with the help of chains and clambering over narrow rocky ridges.

The views from up there were pretty blinding, I'll admit, but it's a welcome reward to also enjoy the mountains from this viewpoint - with the twinkling Seto Inland Sea shimmering between us and sand massaging my feet.

Walk Japan's 10-day Kunisaki Trek is leading us around a little-visited part of Kyushu (Japan's southernmost main island) to follow highlights of a pilgrimage trail filled with Buddhas and stupas hidden in mossy forests, on mountain tops or in rural rice fields. But in a surprising and delightful detour, we've been given 24 hours to explore Himeshima Island, 20 minutes north of the Kunisaki Peninsula by ferry. Roughly seven kilometres long and three wide, it's formed from seven volcanoes linked by sand bars and is central to Oita Himeshima Geopark, an area protected for its geological significance.

Yahazudake, a lava dome with a visible crater dip at its summit, is the island's highest point at 266 metres. Climbing it with our tour guide was one option on offer for the afternoon but I've chosen to go my own way, picking up a rental bike - a cruiser in surprisingly good nick considering the $3 price tag, for two hours - for a magical mystery tour around the island.

There are few things that'll more readily put a smile on my face than the act of exploring, especially when the day is still and sunny, and the place so tranquil I feel like I have it all to myself. Lodged in my front basket is a map and though it's in Japanese, my progress can be tracked thanks to pictorial representations of the island's many attractions: a giant butterfly for where hoards of chestnut tiger butterflies amass in May, June and October, giant prawns for the several tiger prawn farms, a lighthouse, hot springs, a temple perched on a tiny rocky peninsula, and more.

The road starts flat and hugging the coastline, with the green bulk of Yahazudake on one side and blue sea on the other. The smattering of English signs are helpful, the first of which draws my attention to Takanosu Sea Cliff, a pale eroded cliff pocked with holes enjoyed by nesting falcons.

Interesting geosites such as this are a regular occurrence on Himeshima. Faults and folds crumpled by volcanic action are visible, and 40-metre high obsidian cliffs were once quarried for stone tools, but perhaps the most exciting geo feature (to me, anyway) is the fossilised tusk and bones of a mammoth that I see later on display in the island's small museum.

The first tiger prawn farm appears as a vast sheltered enclosure of seawater aerated with a dozen-odd sparsely scattered paddle-wheels - probably explains the half a dozen fluffy cats loitering nearby. It's a short slog (okay, push) to the lighthouse for views over the Seto Sea towards Honshu, but on the descent back down I discover my bike's breaks sound like a screeching pterodactyl. In such a peaceful landscape, I fear the entire island can hear it.

At Hyoshimizu Hot Spring, on the north coast, I don't stop for a soak in the indoor onsen but do pause at the bicarbonate mineral spring outside - a milky-coffee brown pool fed by clear water at its source. Can I drink it? Yes, according to the sign. Should I drink it? Not sure, but I use the cup provided and slurp it anyway. It tastes like lukewarm Soda Stream. A gentle climb past rural homes and gardens leads me through a mountain tunnel and then the pterodactyl screams again on my descent back towards town. Here I come, everyone!

If my tour de island felt like an authentic rural Japan experience, our ryokan accommodation only cements it. Walk Japan favours traditional local inns throughout our itinerary but Yachiyo-kan is the most traditional yet, a rabbit warren of corridors and steps linking rooms hidden behind sliding paper screens, where beds are rolled out on tatami mat floors and bathing is enjoyed in the steamy communal bath house. Tiger prawns feature heavily on the dinner menu - so fresh, they're served sashimi style or cooked. Himeshima's prawns have been famous since cultivation began in 1963 but before that, the shallow tidal sand flats were used for mining salt as far back as the 1500s. The Kosho family - village headmen during the mid-1800s - constructed some of these salt pans and our guide takes us to visit the Kosho home the next morning.

Regular ferries to Himeshima depart from Imi Port in eastern Kyushu. Travel independently or visit as part of Walk Japan's 10-day Kunisaki Trek which costs approximately $5000 (JPY480,000) and includes accommodation, ferries, luggage transfers and most meals. Qantas and Japan Airlines fly from most Australian capital cities to Fukuoka, then catch the Sonic Express train to the tour start point, Nakatsu. walkjapan.com

The writer travelled as a guest of Walk Japan.