Big Bend National Park's Hot Springs is a natural gathering area, and a place to learn (with your feet!) lessons about geothermal power. If the water level is right, you can put one foot in the cool Rio Grande river, and other in the warm water (105 degrees!) of the hot springs ruin!
Here's the park's page, "Take a Soak in the Hot Springs"
Here's a guide to the history of this special place! This page also has a great suggested reading list so you can learn more about the area!