Most people have heard about the Seljavallalaug pool. Its temperature is lukewarm and some people advise against swimming there because of bacteria/algae in the pool. However if you go a couple hundred meters up the river, you will find a pretty cool makeshift hot pool right next to the river where you can do cold dipping in the glacial river followed by 42C of pleasure.
GPS coordinates 🗺 63.567221, -19.606444