More than one way of doing the Golden Circle
The Golden Circle tour can be done in many ways.
A: You can do it clock wise, start at Thingvellir, continue from there to Geysir, even have lunch there (if you like eating with 5000 others!) and from there you travel to Gullfoss waterfall where you also can have lunch (the lambmeat soup is highly recommended). You drive back through the Kerið crater or through Flúðir where the Secret lagoon is located. Some tour companies and operators do the GC this way. From the Secret lagoon you drive the south coast, road nr. 1 (the ring road that goes around the whole island) back to Reykjavík.
B: You do the same tour as above in A: but you start the tour between 6am and 7am in order to be early in Thingvellir and avoid the crowd. It is really worth it.
C: You do the tour anti-clockwise, allowing you to start the day by a riding tour on Icelandic horses. From there you drive to Kerið and then the same roas as in A:, just the other way around.
See map here:
D: You want to go snowmobiling! In this case you start the tour at 8am, drive the south way straight to Gullfoss waterfall and go with Mountaineers of Iceland at 10am. You´ll be back around 2pm, you go enjoy the Gullfoss waterfall, have lunch and continue to Geysir and from there to Thingvellir. From Thingvellir the drive to Reykjavík takes 40 minutes.
E: To be recommended: Regardless of you doing the tour clock-wise or anticlock-wise, you shouldn´t miss the opportunity to visit the Friðheimar tomatoe farm. It is more interesting than it sounds! (Bill Gates was there). It is a technically very advanced tomatoe farm and restaurant. You can order a presentation about the farm but the waiters explain to every guest having lunch the history of the farm, the use of geothermal power and what they produce of the tomatoes.