Driving from Giants Causeway to the Carrick-a-rede bridge, you come across the ruins of Dunseverick Castle and I would absolutely recommend stopping to take a wee look.
It's amazing to see the structure built with the sea to its back and although all that is left is a couple of crumbling walls and a few foundation stones, the walk down to the natural harbour is really worth it as it brings to life the smuggling tales from earlier times.
One caution however is to be careful because the ground is uneven and it's a fairly steep climb back up to the road, but worth it. Another pity is that the lay-by for this attraction is fairly small and not much parking which means if it's packed, you won't be able to stop.
Note: #226 of Yelp 100 challenge for 2016 read more