Termas de Cauquenes (tel. 02/6381610, fax 02/6322365 in Santiago, www.termasdecauquenes.cl [1]) is 31 kilometers east of Rancagua [2] via the paved highway to Coya and a six-kilometer gravel road to the south. Both the restaurant and individual tubs are open to nonguests. Lunch costs US$18, dinner US$28; an hour’s simple soak costs US$8 per person, while a Jacuzzi costs US$11 single, US$18 d.
Ever since the Jesuits first established themselves at the foothills hot springs Termas de Cauquenes in the 17th century, it’s pulled in prestigious passengers such as Bernardo O’Higgins, José de San Martín, and Charles Darwin. In their day, though, it wasn’t the luxurious retreat it is now—in 1834, Darwin described it as “a square of miserable little hovels, each with a single table and bench.”
In 1876, though, Copiapó nitrate magnate Apolinario Soto modernized the facilities, creating a Vichy-style pavilion with tubs of Carrara marble, and Termas de Cauquenes [3] has retained its elite image ever since. Temperatures in the pools range 42–48°C.
On the Río Cachapoal’s south bank, 760 meters above sea level on beautifully landscaped grounds, 
Hotel Termas de Cauquenes (tel. 072/899009, fax 072/899010, US$82 pp with breakfast, US$125 pp with full board) has pools and tubs, modern conference facilities, and a fine—though some consider it overpriced—riverview restaurant. The rooms are more than comfortable.
From Andén 13 of Rancagua [2]’s Terminal Rodoviario, Buses Coya (tel. 099/4451555) goes five times daily to Cauquenes (US$1.50) between 6:45 a.m. and 10 p.m.
Links:
[1] http://www.termasdecauquenes.cl
[2] http://www.moon.com/destinations/chile/chilean-heartland/-southern-heartland/rancagua
[3] http://www.moon.com/destinations/chile/chilean-heartland/southern-heartland/rancagua/sights/termas-de-cauquenes