Reykjadalur is a geothermal valley hike that brings you to the most epic hot spring river. It is completely free, but you do have to pay for timed parking. The hike is 6 km return that usually takes about an hour each way. The hike is a bit steep and goes up about 200m in elevation on uneven ground. Bring a swimsuit, towel and a dry change of clothes because there are no formal changing facilities, and carry water and snacks since services end in town. The hot river has a range of temperatures and boiling spots, so test water carefully, avoid obvious steam vents and follow posted safety signs; there are no lifeguards and you swim at your own risk. It gets hotter the higher up the river you go. Since it is free, it does get busy! This was a highlight on our trip.