You can make an easy, satisfying 10km circuit of Þingvellir and the rift valley along marked trails in under three hours, more if you like to linger and explore along the way. Starting at the National Park Information Centre, walk a short way east along the road past the campground until you see a marker for the 2km footpath through a landscape of mossy rocks, lava pavements and dwarf birch scrub to Hrauntún. This is the site of an abandoned farm, still ringed by a lava-block wall. There isn’t a single tree on the site – clearing them all must have taken some effort.
Turn south and follow a well-used 3km track to Skógarkot, a sheep farm abandoned in the 1930s, whose ruined but strongly constructed stone buildings occupy a grassy hillock roughly halfway across the valley. It’s not a high position, but still elevated enough for you to take in a panorama of distant peaks and rift walls, and feel dwarfed by the scale of the Þingvallahraun flow which stretches all the way from the lake to Skalbreiður. From here you bear west towards the church at Þingvellir itself – a further 2km along a decent trail – and then follow Almannagjá for a final 3km back to your starting point.