I went with the Mrs last year.
Reykjavik is great, not much more expensive than London really, lots of food, drinks, nice atmosphere, etc. We also went to see the geysers, the blue lagoon and some waterfalls, all good.
Also spent a week bombing around the volcanic landscape on horseback for a week, but that's probably a bit specialist! :-p The landscape is awesome though, get out in to it if you can...