The Senedd, free
Richard Rogers's glass parliament opens to anyone after a quick screening, and on sitting days you can watch a debate from the gallery.
A century ago this was the world's greatest coal-exporting port, and its dockside quarter, Tiger Bay, was one of Britain's first truly multi-ethnic neighbourhoods, home to Yemeni, Somali and seafaring families from everywhere. Then the coal went, the docks sank into mud for decades, and in 1999 a barrage 1.1 km long was closed across the mouths of the Taff and Ely. The tidal mudflats became a calm freshwater lake of roughly two square kilometres, and everything you see today, the promenade, the cafés, the glossy frontages, exists because of that wall. The heart of it is Roald Dahl Plass, a broad oval named after the children's author, who was born in Cardiff to Norwegian parents. Three buildings around it stand more than a century apart: the Wales Millennium Centre (2004), its bronze-coloured steel roof lettered with giant lines of Welsh and English verse; the red Gothic Pierhead of 1897, with a small free exhibition on the port's history; and the Senedd, the Welsh Parliament, designed by Richard Rogers and opened in 2006. Entry is free after airport-style screening, and if you land on a sitting day you can watch a debate from the public gallery. A few steps away stands the white timber Norwegian Church where Dahl was baptised, now a café and small arts space. It works well for families and pushchairs: the ground is flat, almost step-free, and the spaces are wide open, with the hands-on Techniquest science centre built for children. Boats leave from the waterfront below the plaza and by Mermaid Quay: short bay and barrage spins of 15 to 30 minutes, or the waterbus up to Bute Park by the castle, about 25 minutes for £8-9 one way depending on the operator. Mermaid Quay has plenty of restaurants, halal options among them, but be clear about what this is: a modern leisure district, not historic streets. If you want old Cardiff, the castle and the city-centre arcades deliver it; the Bay does not. Come in late afternoon and stay past dusk, when the facades light up and double in the water; in flat daylight the plaza can feel a size too big. In winter the wind rolls in off the Severn estuary and the Senedd and Pierhead shut at 16:30, so between November and March make it a morning outing. From Queen Street station in the centre, trains reach Cardiff Bay station in four minutes, four an hour, then it is a ten-minute walk to the plaza; the number 6 bus (the old Baycar route) also runs down. Drivers have the Q-Park multi-storey on Pierhead Street, five minutes' walk away.
Richard Rogers's glass parliament opens to anyone after a quick screening, and on sitting days you can watch a debate from the gallery.
The Millennium Centre and the red Pierhead light up and mirror in the lake; this is the hour the Bay earns its visit.
Walk or take a boat out to the 1.1 km barrage of 1999: a free promenade on the Wales Coast Path with views over the Severn estuary, a café and a play area.
Roald Dahl Plass, Cardiff Bay
With a single half-day, the castle and the city centre come first; the Bay is a handsome modern district but carries none of the same history. If you are staying a night, make it your evening: dinner at Mermaid Quay, then the promenade with the buildings lit. It is also the easiest part of Cardiff with children and pushchairs.
Prettier, certainly, but slower and dearer: about 25 minutes down the Taff against four minutes by train, sailings are hourly and stop when the weather turns or the river runs high. Go down by train and come back by boat if the weather is fair.