Mulafoss is a very beautiful waterfall, well worth the 4.3km out-and-back hike from the parking next to the Fossalar campsite.
Still somewhat of a hidden gem, this is a place you would want to visit if you are looking for a more relaxed, off-the-beaten-path experience a short distance from the ring road.
Apart from Mulafoss being a very beautiful waterfall, the whole Fossadalur valley is worth the hike. Very peaceful and quiet area.
On Mulafoss; you can hike down to the riverbed and get quite close to the waterfall, and you can also hike right up above it. There are several nice viewpoints all worth exploring!