Águas de São Pedro
Although Águas de São Pedro is one of the smallest cities in Brazil, at a mere 3.9 square km (1.5 square miles), its sulfurous waters made it famous countrywide in the 1940s and '50s. The healing hot springs were discovered by chance in the 1920s when technicians were drilling for oil.
Fonte Juventude is the richest in sulfur in the Americas and is often used to treat rheumatism, asthma, bronchitis, and skin ailments. The waters at Fonte Gioconda have minor radioactive elements (and, yes, they are reportedly good for you), whereas Fonte Almeida Salles's have chlorine bicarbonate and sodium (which are said to alleviate the symptoms of diabetes and upset stomach).
You can access the springs at the Balnéario Publico (public bathhouse) or through some hotels. Though a number of illnesses respond to the water, most visitors are just healthy tourists soaking in relaxation. Águas de São Pedro is compact, so it's easy to get around on foot.