The Golden Circle is a fairly short scenic loop from the Icelandic capital of Reykjavik. It takes in several of the famous attractions of the island and can be done in a single day. Gullfoss (Golden Falls) is one of these attractions. The falls are 35 meters (105 feet) high and thunder in two cascades into a chasm. A Visitor Center is available for information.
Thingvellir National Park is perhaps the most important area along the drive. Here, visitors can experience Plate Tectonics (the theory that the surface of the earth is made up of plates which are riding on a conveyor-belt-like mantle and can collide or interact with one another in various ways) in action, for here the Eurasian Plate, which includes Europe and Asia, is separating from the North American Plate (USA, Canada, Mexico, and part of Central America). Visitors can actually walk down into the rift caused by this separation.
Incidentally, the Plate Tectonics idea explains why Iceland is so active geologically — it was formed from an upwelling of material from the mantle to fill in the void created by the separation of the two continental plates. A more recent example of this action was the birth of the island of Surtsey, just south of Iceland, which began in a series of eruptions that eventually lifted it above sea level in 1963. Recently (April, 2010) the volcano under Ejafjallajokull glacier erupted and "rained" havoc on Transatlantic and European air travel. Other evidence is the fact that most of Iceland’s electricity is produced from geothermal power.
Thingvellir is also a religious site, a place which, in Icelandic lore, had to do with the Guardian of the Laws.
The third major attraction along the route is Geysir, actually the name for a now-dormant geyser which fascinated tourists until 1916 when it abruptly stopped erupting. The current attraction is Strokkur (The Churn), another geyser in the area, which boasts mud pots and steam vents and other geothermal features.
While in the area, be sure to spend some time in Reykjavik itself, one of the most expensive destinations in the world.
Another unique experience in the Reykjavik vicinity is the Blue Lagoon, a geothermal spa which can be visited year round.