The Pontcysyllte Aqueduct is one of the great engineering achievements of the British canal age, inscribed as a UNESCO World Heritage Site. The structure carries the Llangollen Canal across the valley of the River Dee in a narrow iron trough, supported on 18 elegant stone piers. At 307 metres long and 38 metres above the valley floor, crossing it on a narrowboat — with one side open to the air and nothing but the iron side of the trough between you and the drop — is a genuinely vertiginous experience.
You can also walk across on the towpath alongside the boats, which is free and gives the full sense of the engineering. Horse-drawn boat trips run from Llangollen wharf and take about 90 minutes return including the crossing. The aqueduct can be seen well from the valley below, where a footpath runs along the Dee, giving a perspective on the scale that you cannot get from the top. The surrounding Dee valley is a Site of Special Scientific Interest, and the walks on either side of the aqueduct are excellent.