Ramune Onsen-kan is a newer structure with iron-rich water in the town of Taketa, in Oita Prefecture. It boasts rich, mineral-laden carbonated water and beautiful rural scenery next to a river. The carbonation is a unique feature, relatively rare and thus fun to seek out if you're in the area.
Taketa has a train station but the onsen is a good 10 or 15 mins drive away, so you'll need a taxi, ride-share, friend, or your own set of wheels to arrive.