Have a great time (you will!). Top tips - take the lift up Halgrimmskirk (the big church overlooking the city) for a great view of Ryk and surroundings. You have to pay for the galleries but one payment gets you into the main ones and there are some great exhibitions if you are artsy. Wander round inside of Harpa (its as cool inside as it is outside). Eat a hotdog at the harbour (Benji Bestu). Go to Bergsson Mathus for the best breakfast ever. Dont buy anything in the 'Puffin Shops' its overpriced tat. If you have a fancy for something knitted in Icelandic wool (the jumpers last a lifetime) get it in a charity shop for 1/10th of the price of a new one. If you really want a new one get it from the Handknitting Association as these are made by individual local knitters rather than the mass produced ones in the other shops. Kex Hostel has good music and relatively cheap food and a good atmosphere.
a top post for what to do in a day in Reykjavik! We did most of this, barring Kex, and it really was rather grand. the inside of Harpa is well worth calling in on, I had to talk mrs b into it, and she only agreed when a freezing wind blew up off the Arctic, but she was very happy to have done so. Great hotdogs too.
Weather wise, we'd probably have been better off in wellies than in walking boots, a few days that were apparently more like summer than the middle of winter meant that a hell of a lot of snow had just turned to mush. And, as i discovered my right boot leaked a little on day one, that we less than ideal.
but I coped, and so much of it was just amazing anyway, Gullfoss is really rather gorgeous (had to play Porcupine as soon as I got back), and the national park thingy looked lovely as the sun was rising (that was a bit odd, just how dark it was at 10am, I knew in theory it should do, but it still seemed so peculiar). Prices were nothing like as bad as I was expecting, there was a fair bit of very nice Icelandic beer (Gull is a bit shit, tho), and i had one of the very best meals I've ever had). Top stuff.
Could have done without the two hours standing around hoping for a glimpse of the Northern Lights on our last night, tho,but you can't have everything.
Oh, and Icelandic horseracing is hilarious.