I'd add to your list Gullfoss, Vestmannaeyjar, Geysir, and Thingvellir. For the geothermal soaking pools, I've heard the Blue Lagoon is best.

Gullfoss (although it might not be accessible in January)
https://www.google.com/search?q=Gullfoss&espv=2&biw=1920&bih=955&site=webhp&tbm=isch&tbo=u&source=univ&sa=X&ei=KCacVY25MIfWoATUsJCwCw&ved=0CCcQsAQ


vestmannaeyjar (especially if one of their two lava cones are spouting)
https://www.google.com/search?q=Gullfoss&espv=2&biw=1920&bih=955&site=webhp&tbm=isch&tbo=u&source=univ&sa=X&ei=KCacVY25MIfWoATUsJCwCw&ved=0CCcQsAQ#tbm=isch&q=vestmannaeyjar


Geysir (the one all the others are named after)
https://www.google.com/search?q=Geysir&es_sm=122&tbm=isch&tbo=u&source=univ&sa=X&ei=3SacVZvIGMnboAS04bu4Cw&ved=0CDgQsAQ&biw=1920&bih=955


thingvellir
https://www.google.com/search?q=Geysir&es_sm=122&tbm=isch&tbo=u&source=univ&sa=X&ei=3SacVZvIGMnboAS04bu4Cw&ved=0CDgQsAQ&biw=1920&bih=955#tbm=isch&q=thingvellir