The Golden Circle in Iceland is a round trip that can be easily done from Reykjavik. In connection with the Golden Circle always the 3 highlights Gullfoss, Geysir/Stokkur and Thingvellir National Park are mentioned. But also the Brugafoss should not be missing on this tour.
Everything in one day will be quite sporty. Thingvellir National Park is relatively large and can be done separately. It is also less dependent on good weather.
Thingvellir
Thingvellir National Park is the oldest national park of Iceland and also historically relatively important. In Thingvellir National Park you will find the Öxararfoss waterfall. You will also walk through wide fissure. Other fissures are filled with water. The most famous is the Silfra fissure. This crevice was formed by the drifting apart of the Eurasian and North American continental plates. If you want you can snorkel or dive here over tours with dry suit. The water is freezing cold (about 3°C), but also super clear with visibility up to 60m.
https://www.thingvellir.is/en/
Geyser / Stokkur
Here you will find the only true geyser. He was the name giver for all other geysers. But why he of all geysers, I don’t understand. The real highlight is the Stokkur. The Stokkur shoots up a water fountain every few minutes. But if you think you can set the clock after that, you are wrong. At the Old Faithful Geysir in Yellowstone National Park/USA this works with the clock, here it is always a surprise. But it explodes relatively often. He can shoot directly 2-3 in a row or makes sometimes 10 and also sometimes 20min break. But that also means to wait the whole time with the camera not to miss the moment.
Bruarfoss, Midfoss and Hlauptungufoss
You can reach all 3 waterfalls via the Brugatrail, a 3km long hike (6km there and back).
From the parking lot you get to see Hlauptungufoss first. Here you would rather not fall into the water. A narrowing and a small waterfall turns the water into a raging river.
Next you reach Midfoss. This one is relatively unspectacular, but also has a beautiful blue color.
The last waterfall is Bruarfoss, which is not very powerful, but very beautiful. Finally you need to go back the same way.
Gullfoss
Gullfoss, golden waterfall is the namesake of the Golden Circle. A large and powerful waterfall with a lot of water mass. First a small slope, before it then again falls into a gorge. Because of a lot of spray, there are rainbows to see when the sun is shining. You can see the Gullfoss from a low and a high viewpoint. If the sun is high, the light is better because it also reaches the gorge. Late morning or early afternoon are therefore best for a visit.
