With another hike planned for the afternoon, I decided to kill the morning by doing the Merkurrani Plateau hike, which didn't sound all that exciting.
At first it goes up through the woods, past Sönghellir cave, which I didn't stop to look at. Maybe next time. The trail is supposed to turn west, while the trail up to Valahnúkur goes straight. I didn't see my turn, but I wanted to go up it anyway.
It was a steep ascent, 869 feet, and much of it heavily used and in need of some TLC. However, the view from the top was incredible in all directions. At the peak there is a sundial (inoperative at the moment) that also labels all the surrounding features. Unlike Rjúpnafell, the view from the top was much better than on the way up.
On the way down the turnoff was more visible; at least, the pegs were. The path itself wasn't worn. It followed the top of the plateau quite pleasantly for a while, before descending to the valley below.
Supposedly, the trail passes by the cave Sóttarnellir, but I didn't see it. However, I had descended all the way to the valley, and on the map it's not clear the trail goes that low.
The hike was about 3-4 miles total, with 1079 feet of cumulative elevation gain.