What trek should I do in Iceland? I have 5 days and travel with 1 friend
That all depends on what kind of treks you'd like to do. Are you looking for treks for several days or just short half or whole day tours? What time of the year are you planning? The season is short and you should always be prepared for the worst of weathers, no matter the time of year or the weather when you start. That doesn't mean you'll get bad weather, but it can happen any time of the year.
One of the most popular treks in Iceland is Laugavegurinn hiking route in the highlands of Iceland, but you usually need to book that in advance due to popularity. It starts in Landmannalaugar and ends at Þórsmörk or even Skógafoss which will take you across the new lava from the Eyjafjallajokull eruption. That last part can also be done in a one to two day hikes.
There are other places that are nice for hikes, including a glacier hike on Sólheimajökull but you should always have a guide with you when going on glaciers. A safe hike would be Snæfellsjökull volcano in the west of Iceland and includes some beautiful landscape.
In the Westfjords where I have hiked the most you can find endless old routes that were used for centuries. Most spectacular ones are in Látrabjarg, Europe's most western point and largest birdcliff, and around Hornstrandir Nature Reserve in the north of the peninsula but there you'll probably need all of your days :)
Further east you'll find Skaftafell, part of the Vatnajökull National Park with beautiful routes and magnificent landscape, black sands, green forests and white ice. It is not far from Jökulsárlón which is one of Iceland's top destinations.
You can hire a guide for any of those tours or for most of them, just go on your own if you have the right information with you.
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