I was in Reyjkavik three years ago in September for three days. I did a Golden Circle Tour. I think the company was Gray Line. I just looked at a list and didn't see it on there. I feel like it might have been most of the day, something like 9-3. They typically will take you to the Geysir area, Gullfoss falls and Thingvellir. For anyone that has seen Yellowstone, the Geysir area is underwhelming, though intersting. I liked Gullfoss and Thingvellir. I think there are hop on hop off tours, but if there isn't guide, I'd think you'd miss a lot on the history and geology of Thingvellir. It cost about $90 USD so you can take that into account vs. rental car, though I think those are pricey, too. Other than that I walked around town and saw all the sights. Somewhat close to town is the Reykjanes peninsula which has a lighthouse and lots of birds. I learned too late that I could have taken a city bus there. Make sure to walk along the harbor and go into Harpa, and see the building where Reagan and Gorbachev had a summit. I went to the top of Hallgrimska where the views were amazing. There are fun buildings to look at and lots of quirky graffiti. Food is pricey although grocery stores are reasonable. It was fun to have a hot dog at Bæjarins Beztu Pylsur, where a lot of famous people have stopped. I also thought the food was good and prices ok at Icelandic Fish and Chips. I didn't go to Blue Lagoon, its really pricey and its man made. Alcohol is super expensive. The best place is to get whatever you think you might want at the Duty Free before you leave the airport.