Er, so much I could tell you ... but first, a couple of top tips....
If you like to relax with a drink, bring your duty free allowance INTO the country!!! And despite what the posters say in English, the allowance is one litre of spirits and three bottles of wine (or at least, that's what they say in the Icelandic translation).
It is true, beer is #5 a pint.
The cheapest way to spend money is on your Visa card - the exchange rate is better than for cash. And everywhere takes cards (using cash implies you are too poor to get credit!!!!)
Er,lots I can tell you - what does the break include?
Basically..... Reykjavik (where I assume you are staying) isn't up to much, but the countryside around is. There are lots of trips that you can take (if there's a group of you it is cheaper to hire a car), there's a list of excursions on http://www.re.is/tripsList.asp?cid=1
Any questions, just ask away ....
www.visitreykjavik.is has loads of useful info as well.