We stayed at a local camp site during golden week 2015, which was just a 15 minute drive from Ojiragawa Canyon.
We stopped here on the way back from a day trip to Matsumoto castle (also recommended). We didn't expect too much of our visit to Ojiragawa Canyon but it was so stunning we just loved it. The walk was lovely and gentle. The car park is free and the walk starts from here. We took our 5 year old and he didn't find it too difficult.
The start of the walk takes you around the top of the canyon and the drop is quite big so hold the hands of any young children. Then you'll pass through a beautifully well kept temple. The temple have left branches of trees that can be used as walking sticks for the hikers and day trippers which was so kind. Many young children and older people used them along the walk and returned them on the way back.
The walk takes you over a suspension bridge and over the river at the bottom of the canyon. Only 5 people at a time are allowed across. Then we walked to the river at the bottom of the canyon and sat on one of the rocks. Children and adults were wading through the crystal clear water. Stunning.
At this point we turned around and went back as our 5 year old had a long day, as we combined this visit with a visit to Matsumoto earlier in the day. You can carry on the walk much further. I'd recommend a half day here. Well worth it to explore more of this lovely area.
On the way back from here about 15 minutes drive from Ojiragawa Canyon there is a day onsen/hot spring spa. Really lovely and recommended to soothe your muscles after the walk. The name in Japanese is 尾白の湯 Verdant Gardens Onsen.