Thanks to Icelandair (seriously, no sarcasm), I had a long layover in Reykjavík on my way home, which allowed me time to get to the Blue Lagoon. It's actually just a quick bus ride from the airport, and you can get tickets online ahead of time. (You actually should do this, as spaces to visit fill up well in advance.) I planned this little day trip well before I left Madrid, and it was the perfect way to relax and pass the time on a long journey home.
The geothermal pools in the Blue Lagoon are actually man-made, which is a common misconception, and are filled with runoff from the nearby geothermal plant. The silica in the water is what gives it a chalky color, and then the reflection from the sky makes it appear blue.
The entire pool is surrounded by lava rock, which just blows my mind. (Hey, I'm a small-town girl, okay.) You can see the white from the silica that have stained them over time. There are swim-up bars where you can get silica masks, and you will see that most bathers do this. You just rinse them off with the water after 5-10 minutes, and they make your face feel super smooth.
I did not get my hair wet while in the pool, because I had several hours of traveling to do, but you should apply the hair conditioner that is in the restroom if you want to do that. Otherwise, the silica can damage your hair. There is no need to bring a bathing suit or towel if you don't want to, as you can rent them there. There is a cafe, restaurant, spa, steam room, and all the space to bathe for hours if you like. Venture throughout the pools to find the warmest spots and relax!
The Blue Lagoon is a must-see stop if you're ever in Reykjavík, even for just a layover. There were all kinds of people there, from families with young kids, to older couples, ones on their honeymoon, and even a bachelor party. What is most authentic about it to me is that Icelanders regularly go there as well, together or alone, to relax. It's well worth the money if you ask me.