Cardiff Bay is about 2 kilometres south of the city centre and makes a good half-day excursion on its own. What was once the Tiger Bay docklands, one of the world's busiest coal ports in the early 20th century, has been transformed into a waterfront plaza ringed by restaurants, cultural buildings and converted warehouses. The atmosphere on summer evenings, with the lake lit up and the Millennium Centre's facade glowing, is genuinely impressive.
The barrage itself — a long walkable dam across the mouth of the bay — is worth crossing for the views. On one side you get the freshwater lake and the city skyline; on the other, the open Bristol Channel. It takes about 20 minutes to walk the full length.
Getting there is easy: the Baycar bus runs regularly from the city centre, or it is a pleasant enough walk through Butetown following the waterfront trail.