I love onsen and I have been a couple times to Kurokawa before finding out that there's more in the neighborhood than just the Kurokawa area. If you're coming from Kumamoto city, you'll probably pass through Minami-Oguni Town. It's not as touristic as the onsen area but great for a stroll or a small hike. The thing I like about Manganji-onsen is that it's really local and out in the open, connected to the local canal/river. The engineer of this piece of art divided different tubs for people's bathing, vegetable cleaning and animals' washing! Maybe not the best onsen if you're expecting prepared shampoo, conditioner and air conditioned changing rooms, but definitely worth a try as footbath. Also, in the vicinity of the onsen there are hiking trails and farm restaurants for a yummy lunch or dinner.