Thingvellir, Geysir and Gulfoss are all very beautiful sights and highly recommended. They are busier than the south coast, simply because they are so close to Reykjavik but we didn't struggle with getting a parking space at any of the locations. Thingvellir is only 35 minutes from the City, Geysir is a little further on (about 25 minutes from Thingvellir) and Gulfoss is about 20 minutes from Geysir. They are all really easy to find (even without a map) as they are all signposted.
If you own or manage Golden Circle Route, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.