Asahi Onsen has two of my favorite baths that go great together, the 46 degree extra hot bath and the refreshing 31 degree hot spring bath. The onsen here is much like that of Minatoyama Onsen or Nada Onsen to name a few. It has a metallic taste and feel. Cooling off in the cool spring water bath after sitting in the 46 degree hot bath is one of the reasons I am addicted to this place. I travel the extra distance to get here, and other places like it, for the experience. There is a little balcony with an open air cold water bath and a good selection baths and saunas in its third floor bath for its size. Since the changing room and entrance is on the second, you will have to climb some stairs naked.
Kids will play around in the water and enjoy themselves. I've had a number of funny conversations in the changing room with the other bathers here. It is a great community bathhouse with an outstanding spring.
Review Update January 2020:
One of the best sento in the country. It has a true hotspring feel to it like it was in Kagoshima or someplace far away from the big city--not a short walk from JR Kobe Station. The extra-hot 46℃ Kuro no Sen (黒の泉) is a perfect complement to the straight from the well gensen kakenagashi (源泉掛け流し) lukewarm bath or the outdoor cold water bath. The lukewarm gensen bath can be busy at times so take your time here and grab a spot when one opens.