On our Iceland trips, we really enjoy the lagoons/hot springs and since there are always so many questions about these, I thought I would share our experiences and some details of the commercial ones. There are also plenty of wonderful "rustic" ones around the country as well as all of the local pools in each town. This is of course not a comprehensive comparison, but rather our experiences at the various lagoons and some details. We have been to some of these multiple times.
**The biggest detail for any lagoon is to shower properly. This does not mean the rules do not apply to you because you are modest, etc. It means you shower naked, without your suit on, and get everything clean, THEN you put your swimsuit on. Even if a lagoon does not have private showers, you need to do this. Nobody cares what you look like and people are not looking at you. The lagoons do not have chlorine etc, so this is really important. You are a guest in Iceland and need to respect the rules**
For ladies suits, I find getting my swimsuit wet before trying to put it on while wet is easier, vs a dry suit. My husband says swim trunks are easier to put on dry.
Blue Lagoon--Only about 20 minutes from KEF so is perfect for right after arriving or just before departing. I personally prefer doing this before departing so that I am not so relaxed upon arrival that I want to go to bed. Being relaxed before the flight home is perfect for me. We enjoy the masks. The showers are mostly private stalls, but there are open ones as well. There are spigots throughout the lagoon so you can stay hydrated. The swim up bar has quite a few offerings, including Krapp. Swimsuit rental is available and the suits are quite nice. I had to rent one once as I could not find my suit in my bag, and I was impressed with what they had. There is a sauna as well as a steam room, but we have never used them so I can't comment. Prices start at about $95, but there are several levels to choose from so prices/amenities can vary quite a bit. Due to the silica in the water, it is best to take any silver jewelry off. I wear gold and have never had an issue. For your hair, it is best to wash it and slather the conditioner on and then put it up in a bun. I have done this and accidentally went under water and my colored hair was just fine.
Sky Lagoon--This is about a 15 minute drive from the center of Reykjavik. IMO this lagoon is great for any day of your trip. Upon arrival, it is close enough to the heart of Reykjavik that you can go to your hotel and get settled and then come back here after that or in the evening. It is also great for just before heading to the airport to fly home. If you are basing in Reykjavik, then you can fit this in wherever you have some extra time. You can get there by bus, but have to make a transfer and then walk quite a ways. A taxi is about $30-35 from the center of Reykjavik. You can also buy a ticket that includes transportation which is via Reykjavik Excursions. Book through Sky Lagoon for this. You can opt for private changing rooms or the shared. We always opt for the shared. You need to take your shoes off before you even enter the locker room. The lockers will hold quite a bit of luggage, but if you need more you can store your bags for a fee. The showers are almost all private stalls, but there are 2 open showers. There are towels avaialble when you return to the locker room so you don't need to take one with you. The 7 step ritual has been expanded over the last couple of years and is much more spacious and flows better than it did in the early days of the lagoon. The locker rooms have hair dryers and plastic bags for your swimsuits, but no swimsuit spinners. Prices start at about $135 and this includes the 7 step ritual. The minimum age is 12.