Thermal lagoon with better views than the Blue Lagoon near Reyjavik (you look over the town and towards Lake Myvatn) and half the price. Also steam baths etc.
The Myvatn Nature Baths are magical, but they are hundreds of kilometres from Reykjavik as the crow flies, on the other side of Iceland, so could never be described as a city destination! They are, however, near to the tiny town of Reykjahlid (don't know how to get my keyboard to reproduce the Icelandic characters properly). The Myvatn area is spectacular and otherworldly. We spent happy hours in the Baths surrounded by the snowy landscape.
Myvatn Nature Baths are magical, but they are hundreds of kilometres from Reykjavik, on the other side of Iceland! In no way could they be described as a city destination, situated as they are in the Lake Myvatn wilderness. I absolutely recommend a visit, but don't expect to find them overlooking Reykjavik or you will be sorely disappointed!