The town of Hveragerði resides just 45 minutes east and slightly south of Reykjavik. This is the starting point for a 3-hour, 6 km round-trip trek that's become, like most of Iceland's natural features, more and more popular over the years. Head north along the well-trodden path and you'll eventually find yourself in beautiful Reykjadalur Valley, known for the boiling geothermal springs that give the area its distinctively smokey appearance. At points along the trail, cold water from a nearby river mixes with the steaming water from the hot springs creating, you guessed it, perfect pools in which to relax in relative serenity (when compared to, say, the Blue Lagoon) and enjoy the fruits of your adventurous labor. For a 250 meter overall incline, that's not a bad trade-off.
On a precautionary note, it's important to remember that Reykjadalur, while incredible, isn't without its inherent dangers. Some of these springs reach extremely high temperatures, and the surrounding ground cover can be deceptively thin. If you opt to go without a guide (which is definitely doable), make sure you stick to the path, and look for other groups of bathers to help identify the pools that are safe for swimming. It's also never a bad idea to consult a trusted local beforehand, just to help get your bearings straight. Then go out there and enjoy a year-round Icelandic all-natural hot tub.