Is that 9 hours between landing and departure? It won't take too long to get out of the airport but you should be back about two hours before departure as all US flights leave around the same time and the immigration (to leave) line can get long. It will take 45 minutes to an hour each way to get to Reykjavik from the airport. So that only leaves you about 4.5/5 hours. I agree - Blue Lagoon is probably your best bet - do the full spa thing and have a meal there.