The second hike I did on Tuesday was special enough I thought it deserved its own post. The canyon is accessible by bus once a day from the Volcano Huts (or, I suppose, by foot). The ride is only 15 minutes via Reykjavik Excursions. (If you get to the huts on RE, as most people do, the ride to the canyon is free.)
South of Þórsmörk is the Krossá river valley. On the south side of it, the Stakkholtsgjá canyon opens up between tall cliffs. This canyon itself is the hike, although there is no trail per se. It's quite flat, but a hiker will have to cross a stream several times. I would recommend hiking poles, and if you know me you know I don't use them! But they are helpful for stream crossings.
The canyon itself is beautiful; my photo above doesn't do it justice.* The walls are vertical, and in many places green with mosses and small plants. The canyon winds a bit, so the view constantly changes.
I was the only one to get off the bus, but I encountered other hikers who had arrived by car or tourbus. Aside from the large group of Americans, chattering away about their lives and seemingly oblivious to their surroundings, the experience seemed to reduce most visitors to silence. With the high walls and the tumbling of the water, the space seemed sacred.
It was probably my favorite hike thus far, although I had a feeling that this was one of those trips where I said that about every hike. If I come back to Þórsmörk, it'll be to do the Fimmvörðuháls or Laugavegur treks, not repeat dayhikes, but I would stay an extra day just to do Stakkholtsgjá again.
* Better photos to come from my camera.