Cooper Cienega, Aguanga, CA, 92536

This watering hole, located in Palomar Mountain State Park between San Diego & Los Angeles, is a tranquil oasis in the southern heat. It is heated so that it doesn't seem to be the superior levitating water, but is nevertheless cold and soothing. The appreciation of architectural craftsmanship is evident. The structural masonry provides a place to store large containers. The process is easy: park & fill. I didn't measure, but according to my intuition, the temperature was quite low.
Here are some basic directions from LA.
90 East
405s
105e
605s
91e
15s
Take I-15 Fwy south, continue on Hwy 76 for approximately 20 mi, then turn left onto South Grade Rd. (S6), continue on South Grade to the top Palomar Mountain. You will find the spring structure within mile 46. It is difficult to miss.
Here's the link for a video of this spring from my youtube.