The Mendoza thermal baths are probably the best pools we have been to so far. While not as natural (They are pools not springs) they definitely have a charm and and sense of fun about them. The access point is a cute town reminisce of a smaller San Pedro De Atacama with plenty of restaurants, bars and tourists around. In the springs you have your choice of boiling hot or freezing cold. There is an unusually powerful water fountain down the bottom section of the pools which tends to get in the way of any complete relaxation, but it definitely forces you to find the perfect spot. Meanwhile, up top there are more than enough hassle free hot relaxation points.
From the town and from Mendoza you can organise extra curricular activities like water rafting and rapelling down the side of a bridge.
If you’re cute little town that force you to relax then we recommend staying the night. But last bus back to Mendoza is at 7 and since the pools close at 6 (locker rooms at 6:30) this gives you more than enough time to browse some shops and have an after bath cerveza.