I may be biased, but this is the best day trip from Osaka. It is a great summer hike because the tunnels are like natural air conditioners. It starts out with a short walk through town.
The surroundings are lush and green.
Some wildlife.
But here is what really makes this hike different. The tunnels! We counted six. In one, you can't see light from the entrance or exit.
We found a GeoCache! I won't tell you which tunnel it was in.
Some tunnels, some green, some tunnels, some green.
And some bridges. The whole trail is quite safe. In the old days the bridges were a bit dodgy.
Near the end you'll come to a spot with places to sit and eat lunch or go down by the river. If you wait until the end of the hike to eat, there is a nice restaurant with wild deer or wild boar.
Keep walking and you'll find the station that will take you back to Osaka.
This map starting in Namaze will get you from the station to the trail head.