Almost on the border with Argentina in the Cajon del Maipo you'll find the Baños Colina hotsprings. Fantastic spot -- it has 9 or so pools which drain sequentially into each other from the source of the spring at the top and are therefore arranged from hotest to just warm at the bottom. The top pool was too hot for me to bare (but not others), but this one the third was PERFECT, we could have stayed in them all day -- and the view was spectacular!