I've been twice, once for a boys' trip and once for a gig. Depending on how touristy you like things, I would avoid The Blue Lagoon and visit another spa instead for a more authentic experience.
There's plenty in Reykjavik to see but as already mentioned, it's really expensive for food and drink. The supermarkets are a strange mix of American / British but as everything is imported, it's quite pricey when compared to home. Nightlife generally starts pretty late so bars etc aren't busy until midnight but there's plenty of good bars in the city.