Just out of curiosity, Why are you heading to Iceland?
Things to see:
Anything that ends in "FOSS". These are waterfalls. Magnificent water falls. Gullfoss, Skulfoss, etc...
Blue Lagoon - this is the body of water surrounding the geothermal power plant in Reykjavik. You will not regret taking a dip in these waters.
The University of Iceland Campus - breathtaking. Gorgeous school.
Geyser - The site all other geysers are named after.
Hallgrimskirkja - Towering church in the capitol. Shaped like a rocket ship.
The girls.
Perlan - Offers a great 360 degree view of the city.
The club scene in Reykjavik is outstanding.
Partake of the local brews if you're a beer drinker.
Althingi - Parliament building.
Hofoi - The former French Consulate building was the site of Ronald Reagan and Mikhail Gorbachev's 1986 summit. Its presumably the most haunted building in Iceland.