Not far from Seljalandsfoss, on the road to Þórsmörk (4WD required), hides one of the most thrilling and adventurous scrambles to a waterfall along a river and through an ancient moss-covered canyon. Although the walk through the Nauthusagil ravine is short (roughly 1.5 km, one hour out and back), you have to be reasonably steady on your feet (unless you don't mind getting wet). It requires walking planks, navigating and hopping dozens of pebbles and boulders, using precarious ropes (optional) and traversing walls with chains.
Those with less balance and fitness may find it easier to wear rubber boots and just walk through the river at various sections. To find it, use the dirt road F249 heading towards Þórsmörk. Around 10km north of Hamragarðar you'll see a sign on the right with hikers on it. Park in the lot here and take the footpath straight into the canyon over streams via rocks and planks. Alternatively, don't go directly into the canyon; cross the bridge to the right, go up onto the canyon wall and use the ropes to descend the canyon's bank (use your judgement as to whether the ropes are secure enough to take you weight; they are not checked).