The National Museum Cardiff is free to enter and good enough to justify most of a day. The Impressionist and Post-Impressionist collection is the headline attraction — Monet's waterlilies, Renoir's portraits, van Gogh's Rain at Auvers, and a Cézanne gallery that would hold its own in any major European city. These works came almost entirely from the Davies bequest and remain one of the great surprises for visitors who don't know Cardiff well.
The natural history section is also worth an hour: the full-skeleton dinosaurs and the evolution galleries are well presented, and the whole thing manages the double act of being genuinely educational while not feeling like a school trip. The geology galleries deal with the formation of Wales itself, which is considerably more dramatic than it sounds.
Upstairs, the art collection moves through Welsh and British painters from the 18th century onward, with a good selection of Welsh landscape painting that makes a useful companion to any trip into the valleys or the coast. The building itself — Edwardian Baroque, fronting Cathays Park — is one of the grandest in the city.