Day 2 we ventured to the Blue Lagoon. We purchased tickets in advance for the first opening of the day, as was recommended. When we first arrived, the parking lot was mostly empty as was the lagoon. We purchased the basic entry pass, although upgrades can be purchased that give you use of a towel, a bathrobe & flip flops. We brought all of our own items & were just fine.
Icelandic pool etiquette states that you take a full shower with soap before getting into a swimsuit. The harsh water of the lagoon dries out your hair quickly, so it's recommended to put lots of conditioner in and tie it back. Also, bring sunglasses and drink lots of water.
There are different parts of the lagoon that are warmer than others, as well as a swim up bar & shallow places to sit. There are also saunas & steam rooms and a waterfall that massages your shoulders.