From the Ring Road you'll see the beautiful high falls at Seljalandsfoss, which tumble over a rocky scarp into a deep, green pool. A (slippery) path runs around the back of the waterfall. A few hundred metres further down the Þórsmörk road, Gljúfurárbui gushes into a hidden canyon.
A steep parking fee has been introduced here, but you may be able to find a free space further up the road towards the turning for Þórsmörk, where there are limited spaces. Buses stop here for Þórsmörk. There's also a cafe and small shop selling souvenirs.