The three primary stops on the Golden Circle route are the Þingvellir national park, the Gullfoss waterfall (meaning "golden falls"), and the geothermally active valley of Haukadalur, which contains the geysers Geysir and Stroker. We saw that and more - our guide made other stops - feeding Icelandic horses, cooking bread in the heated sand, the Faxi waterfall, etc.
The Gullfoss is a delight for all senses - visually stunning, loud, yet calming, the smell of earth and water, the mist is cold and refreshing. Walk another few feet and the view is so different - it is one of nature's beauties. The water comes from glacial melt flowing along the Hvitá river.
In Iceland you can witness the collision of two continents - and it is stunning. The violence created is right there before your eyes - boulders cut in half, huge cracks and fissures. This is where Europe and America meet - and in the "no man's land" between the two plates is the largest lake in Iceland. On its northern shore stands Thingvellir, the historical meeting place of the Althingi (parliament) from 930 until 1798. This is a sacred place for Icelanders and after being here, I can see why. Also, the water from the lake is the water you drink in Iceland - no need to purify - when it enters the lake from the glacier, it has taken 100 years.
On a side note - some of the filming for Game of Thrones (north of the wall with the wildlings) was filmed here - but in winter of course.