Iceland

With only 6 hours, if it's your first time, I would just hop a taxi to Reykjavík, explore the town, see the tall church, meet and hear the people, check out the shops.

With just 6 hours I am not sure I would go all the way to Reykjavik. There are activities nearby that might be a better use of your time. I'm not personally interested in the Blue Lagoon, but many others are and that might be an option for you.

Is that 6 hours that you are free to do something, or 6 hours from landing to flying again? That makes a huge difference.

If you indeed have 6 hours of free time, rent a car and drive the Reykjanes peninsula. It is beautiful, few people, and close to Keflavik for your return.

If it is six hours between landing and take-off, the Viking World Museum is near the airport and fairly interesting.