We visited Kikuchi Gorge. Here's a bit about the place (copied from a web page about the area):
KIKUCHI GORGE -- The headwaters of the Kikuchi River rise from the feet of the Aso caldera rim mountains and rush down a stony ravine surrounded by virgin forest. The water here is acclaimed as some of the purest in Japan. In mid-summer the area is the coolest spot in the prefecture. Leaves turning red and yellow along the river in fall makes the scenery here one of the most beautiful in Kumamoto.