If you are a diver, go for a dive. If not, go snorkeling. I didn’t expect this to be so good, the water is so clear and snorkeling there was easy. The water is 2-4 degrees C so you have to wear a drysuit. But that’s good since they make you float and not feel the cold. Bonus tip: drink the water, it’s so good. You might want to book an early visit to Silfra if you plan on doing everything on the itinerary.