Grotagja is a stop on the dayhike I had really wanted to see and my boyfriend didn't mind taking me there even after he saw it. It's a cave filled with clear, hot water, the size of a swimming pool. It's a very popular stop in Myvatn Park because the cave was used as a background drop for Game of Thrones. Remember the cave Ingritte and John Snow had sex? Well... However in the series the place is really touched up and a waterfall magically appears. But it is still a wonderful view, the bright blue water in a cave. A shame you aren't allowed to swim!
What a travel guide won't mention is that if you climb to the top of the cave (you see the ropes on the outside of the rock, don't make up your own route), you can see the fault line between the Eurasian and American continent. Quite impressive!