Perhaps the best onsen close to Osaka, Arima Onsen offers two main baths -- Kin no Yu (Gold Bath, 金の湯) and Gin no Yu (Silver Bath, 銀の湯). Kin no Yu has nice, hot rusty ochre water due to a high concentration of iron and other minerals; whereas Gin no Yu has water of a more unusual type: Radium. Though locals will swear by its healing powers, treat Gin no Yu (like all onsen!) with some respect, and decide for yourself whether you're up for self-dosing with radiation therapy.
Watarase Onsen is a large, relatively new onsen, restaurant, and hotel complex on the site of Watase Onsen spring. While it lacks the authenticity of some of the nearby onsen (which have been around for centuries), Watarase Onsen offers bathers some treats that should not be overlooked. Its expansive 6-bath rotenburo system is said to be large enough for 1000 people to bathe, though you'll likely feel like you have plenty of room even if there's only 100. Better yet, Watarase Onsen is one of the few places to offer higaeri (day use) kashikiriburo (private, reservable) rotenburo (outdoor