We did the eight-hour "golden circle" ship's tour out of Reykjavik a few years ago. It's still my favorite excursion, hands down. I pooh-poohed the power plant visit, but it was outstanding. Steam vents everywhere, and the building's architecture was awesome. And I loved walking between the two tectonic plates in Thingvellir National Park. Whatever you do, you'll enjoy Reykjavik.