Went to the park specifically to see Svartifoss waterfall. No admission fee. The path to the falls is easy and either paved or well worn. Plan on a couple of hours round trip. The waterfall cascades over basalt columns. If you want to get close to the falls, visit in low season; there can be 200+ people there in high season.
This review is the subjective opinion of a TravelPod member and not of TravelPod.com.