Contrast bathing (sometimes referred to as hot/cold immersion therapy) is a type of hydrotherapy that I find most people have not specifically heard of, but probably intuitively know about. A contrast bath is more or less exactly what it sounds like: submerging your body (or more commonly, a limb) into two contrasting high- and low-temperatures of water, one after the other.
Simple enough, but is there a catch? No! It is cheap, easy, and can be quite soothing. Contrast bathing may be helpful for enhancing circulation; lessening aches, pains, and stiffness; as well as helping to boost recovery times after a workout or athletic event.