The Golden Circle is a marketing name for three stops that fit in daylight. From Reykjavík the loop is roughly 230 to 300 kilometres, depending on where you turn back. Drive Þingvellir first, then the geothermal field at Geysir, then Gullfoss, then home on Route 35 and Route 1.
Þingvellir
A rift and a parliament. The Alþingi first met here in 930. You walk between basalt walls where the North American and Eurasian plates pull apart. Öxarárfoss is the small fall on the site. Þingvallavatn is the lake. Stay on the paths. The moss takes years to recover from a boot.
Geysir and Strokkur
Geysir itself rarely erupts now. Strokkur, a few metres away, throws a column every few minutes. Stay outside the ropes. The crust is thin and the water is boiling. The field smells of sulphur because it is a field of sulphur.
Gullfoss
Two steps on the Hvítá, dropping into a canyon. The lower path is wet and ices in winter. You can see both drops when that path is open. The river is glacial. It runs grey-white even in sun.
It is a loop, not a pilgrimage. Three named places, a car, and the light you actually have.
Coach tours do this circuit in a day from the capital. Self-drive does too. Either way, the useful fact is the distance, not a promise that you will have the falls to yourself.