I went last November, didn't do the northern lights as wasn't there long enough but may be going back again this year for that reason.
booked a tour where we took in the waterfall at gulfoss which will prob have a bit of frosting in December and look awesome i'd have thought, did the geo thermal power plant and the geiser on this tour too.
There was a restaurant there too which had something to do with Gordon Ramsay, we had a little sample of Icelandic food there: Whale, Puffin, Dried fish and decomposed/fermented shark (which is absolutely horrible).
Blue Lagoon is a must.
They do Whale watching tours too I think but that may be seasonal.
It's a very small town/city mind probably smaller than Swansea. Beer isn't cheap either.
how many days you going for?