St Fagans National History Museum is one of Wales' essential museums, an open-air collection of buildings moved from across the country and rebuilt in the grounds of St Fagans Castle. Farmhouses, chapels, shops, workshops, and workers' cottages turn Welsh social history into something you can walk through.
It is excellent with or without children because the scale feels like a village rather than a conventional museum. Allow several hours, wear shoes for walking, and do not treat it as a quick Cardiff add-on; this is one of the city's headline sights.