Iwaonupuri (イワオヌプリ) is a peak in the eastern part of the Niseko Mountains (ニセコ連峰, Niseko-renpou), a mountain range stretching west from Niseko-Annupuri (ニセコアンヌプリ), the massively popular skiing destination. Iwaonupuri's name comes from the Ainu iwao-nupuri, meaning 'sulfur mountain'. True to its name, here and there amid the volcanic ash littering its slopes, you'll find small yellow crystals of sulfur mixed in. Niseko-Annupuri sits just to the east, while the hot springs at Goshiki Onsen (五色温泉) lie to the south; and indeed, a hike that starts and ends at a hot spring is about as good as it gets. Making the long traverse westwards is also a great day to spend a day in the mountains.
From the trailhead at Goshiki Onsen you'll cross the stream Niseko-Anbetsu River (ニセコアンベツ川, Niseko-Anbetsu-gawa) before climbing up into fields of wild rosemary, crowberry, Japanese wintergreen, and akamono wintergreen. In the summer and fall this area is bustling with tourists. As this is a quite young volcano, there are almost no trees to be found here.
Once you've climbed the steep hill you'll arrive on a gentle slope of bushes laced with Middendorff weigela and Albrecht's azalea, and come shortly to Iwaonupuri junction (イワオヌプリ分岐, Iwaonupuri-bunki). Here the trail splits off towards Nitonupuri (ニトヌプリ) and the marshes of Onuma (大沼, Oo-numa).
From here you'll climb further around big bluffs and up a steep hill towards the summit. You'll pass through a belt of dwarf stone pine before coming abruptly to a desolate field of volcanic scree, a whole world away from the green mountainside you just left. In winter, the Niseko Mountains get the most snow of any range in Hokkaido, so even in June it's possible that there will be snow in the horseshoe-shaped crater.
From here the you'll proceed around the whole rim of the crater--a fairly leisurely stroll. The rim is wide and the ground is relatively featureless, so be careful not to accidentally follow someone else's footprints off the trail, especially if it's foggy or visibility is otherwise inhibited.
Around the summit of Iwaonupuri you should be able to see on the bare earth the signs of erosion at work. The summit is wide and clear, so you can see Niseko-Annupuri in the east and the rest of the Niseko Mountains in the west. Because the hike itself is so short, it's a great one to take the family or beginner hikers along on.
Finishing the loop around the crater, you'll arrive back at the trail you came up--head back down the same way and wash the sweat off at Goshiki Onsen.
Even by the end of June there will likely be a lot of snow. In mid to late June, the leaves will blossom, joined by the bead lily, Albrecht's azalea, Middendorff weigela, and so on. The leaves start to change around the beginning of October. When it's quieter in late autumn the mountain is particularly moving. In winter the whole Niseko range becomes a mecca for ski touring, and you'll be more than likely to find Australians shredding the gnar.