We did the Blue Lagoon on our first ever cruise which was actually a three day Carnival. We thought the Blue Lagoon was great. Perfect water, nice beaches and TONS of hammocks. It was a very relaxing day.
Since the strawmarket is right by the pier it's likely that you have some time to look around. Lots of stuff made of straw. Nothing we were interested in. We just went because we thought w should (been there, done that kind of thing). We walked up and down two aisles and then headed back to the ship.