Deildartunguhver is a hot spring, one of the largest ones in the world by flow volume. It produces 180 liters per second of water at 97 degrees celsius. The hot spring provides hot water for the nearby towns as well as several greenhouses in the area.
Deildartunguhver is easy to reach by car from Reykjavik - take the ring road 1, and then take the exit leading to road 50 near Bogarnes. Drive until you see a sign on your left indicating a tourist attraction (⌘).
See the hot springs, take a few photos. Observe the crystal-clear water gushing out of the rocks.
There isn't a whole lot to do here.
Some locally grown tomatoes may be available for sale.
You can continue on road 50, and you will eventually get back to the ring road.