Lambafellsklofi is a spectacular place to visit: a cleft straight through Lambafell, a small mountain/large hill/rock (we have no definition of when something stops being a hill and becomes a mountain, and such a feature is often called a “fell” or a “stapi”, although the former can also be found in the names of genuine mountains).
The cleft passes straight through Lambafell and is just wide enough to make it possible to pass through to the other side. It is so narrow that I could, where it is narrowest, touch both walls without fully extending my arms, and only at the lower (western) end is it too wide for a tall man to touch both walls at the same time. It only takes about 20-30 minutes to get there by car from Reykjavík and the hike, a circular route, only takes about 40 minutes, less if you are a fast walker. You do have to be able to handle a steep slope, as the slope of the ascent (or descent, if you approach it from the east) is, in my estimation, about 45°degrees, albeit not very long. The knobbly walls of the cleft provide good hand-holds for the climb.