Mount Yari Trail is a 24.4 mile moderately trafficked out and back trail located near the city of Okuhida-onsengō-nakao that features a river and is rated as difficult. The trail is primarily used for hiking, walking, nature trips, and birding and is accessible from May until October.
Beautiful hike that starts off casual with several miles along Azusa River without much elevation gain. After the first half it gets steeper, and the last 1 or 2 miles up to Mt.Yari itself are VERY steep with many switchbacks. The trail is well maintained with lots of signage in Japanese and English.
Getting up all the way to the peak requires scrambling, but there are ropes, chains and ladders that help. You don't have to be an experienced climber to do this, but you have to be sure-footed.
Depending on your fitness you will need at least 2 days to finish this hike; i highly recommend staying in or camping at Yarigatake Sanso, a mountain cabin at the foot of Mt. Yari. The early morning views are spectacular, you might even see Mt. Fuji on the horizon.
There are 2 other cabins with campgrounds along the way, so it's possible to turn this into a multi-day hike, maybe combining it with one of the other peaks in the area.