"Dettifoss is a waterfall in Vatnajökull National Park in Northeast Iceland, and is reputed to be the most powerful waterfall in Europe." https://en.wikipedia.org/wiki/Dettifoss
"It's said to be one of the most unearthly places on earth and quite spectacular.
Hverarönd is an area near Mývatn in the northeastern part of Iceland. The area has a lot of geothermal energy which shows itself in the form of bubbling pools of grey mud and sulphur. As a result of this the area smells rather strongly of sulphur but experiencing the almost alien-like landscape is well worth the smell." http://www.planiceland.com/vtours/myvatn_hverir/
Grjótagjá is a small cave that was formally used for bathing as it has a geothermal spring feeding it. However, the water became too hot to swim in and is now a photographic destination.