Apart from friends and family, there are not many things I miss from Iceland. However, swimming in Icelandic swimming pools gives you a unique feeling of pleasure, different from anything else. I miss that sensational feeling.
Bathing in the geothermally heated water is a luxury not to be missed if you travel to Iceland. Chatting to the locals in the hot tubs is recommended as well.
Three useful links if you want to go swimming in Iceland: