Water from underground springs finds its way into the swimming pools of the Avalon Springs Hotel. The water gushes from a rock face underground at 43°C, and is renowned for its healing properties. The pools are commercialised (including waterslides) and can get unpleasantly busy on weekends and in school holidays. Try to visit during off-peak times (when it's also cheaper).
A great way to get here is to hike 2.4km along the Badskloof Trail (R36) from the car park at the end of Tanner St.