If you don't mind the humidity, you can simply leave the water in the tub until it gets cold. The water in the tub becomes cold by exchanging its heat with the

If you don't mind the humidity, you can simply leave the water in the tub until it gets cold. The water in the tub becomes cold by exchanging its heat with the ambiant air. That's of course assuming you're in a climate where you want to heat the air.