The whole area around Lake Myvatn is a hotspot for volcanic activity.
Here are some of the best places to visit.
Namaskard Geothermal Area
A fascinating area with fumaroles, sulphurous mud pools and screeching steam vents. There is a marked trail and boardwalks, which you should not stray from for safety reasons. Be prepared for the stench of hydrogen sulphide!
This is the site of the recent Krafla fires (1977-1984). The area has recent lava flows, sulphurous steam vents, mud pools, hot springs and an explosion crater. There is also a geothermal power plant with a visitor centre.
This conical volcano is quite easy to climb, and you can walk around the impressive crater rim with fantastic views.
An area of unusual contorted lava formations including arches, caves, keyholes (like the one in the photo), towers and pillars. There are marked paths around the site.
Lake Myvatn shore
The lake itself is beautiful to walk beside or drive around, with clusters of pseudocraters, stack-like formations, hot springs and in summer thousands of wildfowl.
These are just a few suggestions. There is so much to explore in this lovely area that you will definitely want to stay a while.