There are many geologically active places spread all across Iceland. You should be able to find something interesting to watch during your trip. One easy place to go is a small town called Laugarvatn it is right at the shore of a small lake.
We get there by driving along some small roads until we arrive at the small village. The village only got a few houses close to the small lake. I figure you need to take a bit care if you go swimming in this lake since some of the water close to the lake is bubbling. The water here is going up from deep underground and is at boiling temperature – and if you only put your shoe in the sand and move it around a bit hot water will fill the hole you have just made.
There is also a small plant which is harvesting the geothermal energy around the lake which is just one of the many places around this island where you get cheap renewable energy from the underground. We look at the steam for a while before the trip continues around the Golden Circle.