The Giant's Causeway is Northern Ireland's only UNESCO World Heritage Site and its most visited attraction — around one million people a year make the trip. The causeway itself is a mass of roughly 40,000 interlocking basalt columns, most hexagonal, forming a kind of giant stepping-stone pavement that descends from the cliffs into the sea. The precision of the geometry is startling even when you know the geological explanation.
The site is owned by the National Trust and has a visitor centre at the top of the cliff with good geological exhibits explaining the volcanic history. From the centre a path leads down to the causeway itself, which you can walk on and clamber over — the columns are stable and the access is open. The full coastal walk following the clifftops east toward Benbane Head is spectacular and much less crowded than the causeway itself; allow two to three hours for the round trip.
Summer brings heavy crowds, particularly at midday. Go early (the causeway is accessible before the visitor centre opens) or in the late afternoon, and park in the National Trust car park. The Old Bushmills Distillery is 3 km south and makes an obvious pairing.