Iceland's well-known natural geothermal spa is actually a man-made attraction (but don't expect to find that fact on their website). Despite that and its crowded theme park feel, the Blue Lagoon is still a cool and sort of mystical site to experience. Imagine being on the moon, where steam billows out from ethereal, milky blue water and nothing surrounds you for miles except a lava-rock landscape. Now just remind yourself you're in Iceland, that's what it's like. In addition, the healing properties of the water's distinct mineral composition and advanced natural marine cosmetology research make this quite a unique place.
When to Go: Best during the summer months between June and August (but also the most crowded). Though the lagoon is open year round and the water temperature a steamy 100º, winter swimming is for those willing to endure the cold once you come out of the water soaking wet.
How to Get There: There are tons of tour companies that drop off travelers by the busload from the airport and Reykjavik center, but a better option for the more independent traveler is to rent a car from Reykjavik (40 minutes) and follow your own schedule, especially if you want to stay late.
P. S. Don't bother to stay overnight at the Lagoon's Clinic hotel, it's a crock.
Of Note: I didn't catch sight of it, but word is that there's a Lagoon Bar with bikini'd servers where you can order special "lagoon cocktails" as you wade through the mineral laced waters.