The Gullfoss waterfall is inside the canyon of the Hvítá river ("White River" in Icelandic). As a visitor, this is a location you should definitely add to your Iceland activities list. Everyone I know who has been there was stunned by the beauty, size, and sheer power of Gullfoss.
The Gullfoss waters follows the river's crevice and suddenly thunders down two tiers 105 feet wide, in near-perfect, albeit unusual right angles to the river.
Depending on the weather in Iceland at the time of your visit, you will see multiple rainbows at once above the three steps of the river. Three of the most popular guided tours in Iceland include the Gullfoss waterfall in their itinerary: