Reykjadalur Hot Spring River

You park above Hveragerði and climb about 3.5 km into Reykjadalur, the Steam Valley, past bubbling mud pots and hissing vents, until the trail levels out at a stream that runs warm. A wooden boardwalk follows the bank, with simple changing screens and steps down into the water. The river sits around 36 to 40°C, hotter upstream where the springs enter and cooler as you move down, so you can pick your spot.

Bathing is free; the only cost is parking, about 1,000 ISK for up to five hours through the Parka app. The walk takes about an hour up and a little less down, and the trail is moderately steep and muddy after rain. Stay at least two meters from the boiling springs along the way and keep to the marked path.

There are no toilets or bins past the service center, so carry everything out. The honest downside: this is the most visited wild soak near Reykjavík, the changing screens offer little privacy, and the trail closes at times for mud or vegetation recovery.