During winter, you have to take the trail from Kanbayashi Onsen to Jigokudani since the road from Shibu Onsen is closed due to heavy snow.
To get to Yaen koen, it takes about at least 30 minutes on foot, yet it is depends on the condition of the trail. This mountain pass is narrow, unpaved, often covered with snow and full of steps, it is almost impossible to follow by wheelchair.
It is so beautiful to explore winter forest, but you need to prepare warm outfit as well as snow boots.
It was quite a bit of walk from Kanbayashi Onsen through the forest pass way, yet if you are lucky, be able to encounter Japanese Serow (goat- antelope recognized on IUCN Red List for least concern mammals) on the way to the park.
Or you may see some monkeys interrupting some tourists enjoying Onsen Rotenburo (outside bath) at a B & B called Korakukan.
More detail ⇒ Jigokudani Yaen Koen