If you’re completing the Golden Circle in the traditional direction, then the route from Gullfoss back to the Ring Road (Rte 1) at Selfoss will be the final stage of your trip. Along the way you’ll find plenty to lure you to stop. Most people follow surfaced Rte 35, which passes through Reykholt, with its river rafting. You can also detour slightly to Flúðir with its geothermal greenhouses and hot spring, and Skálholt, once Iceland’s religious powerhouse.
If you'd like to continue east rather than return to Reykjavík, the western Þjórsárdalur area is the next valley of interesting sights.