I visited Hjalparfoss with a British group in 2008. We had been to many of the famous large falls and Gullfoss is still top of the list for sheer overpowering grandeur. BUT Hjalparfoss is so beautiful and peaceful. We just sat on the grassy slope, ate our sandwiches and soaked it up. It’s wonderful, And you can visit the restored old farm house just round the corner (in a car anyway). I think that’s called Thjorsardalur, and it was excavated after being buried by an eruption of Hekla about 300 years ago.
The 12th century farm nearby is called Stong and is well worth a visit.