A great place for some short hiking around Myvatn is the area called Dimmuborgir. It is a strangely looking landscape which was formed by lava flowing across a marsh a couple of thousand years ago. Apparently the lava flowing over the marsh caused the water in the marsh to boil. As a result some strange pillars of lava were formed where the boiling water tried to get to the surface of the lava flow. Today you can do a walk of a couple of kilometers across this landscape and look at the strange lava pillars.
There are also some old lava tubes which are more or less collapsed today but there is some still to be seen one of the more impressive lava tubes are called kirkjan or the church.
It isn’t just tourist which enjoys the landscape apparently the Dimmuborgir has been used by HBO as a scene in Game of Thrones.