If you don’t know any better, Iceland may seem like little more than a layover on the way the mainland Europe. But the rewards are great for those who dare to venture beyond Reykjavik’s Keflavik airport, even if only for a few hours. The small island nation’s tourism industry is in high gear—ready for the onslaught of eco-tourists and adrenaline junkies who have already made landfall— but the Blue Lagoon, a geothermal spa just a …