Southbank Centre
Large 1960s development on the South Bank. Built largely of concrete, views on its architectural merits vary considerably. Presents a varied programme of music, literature, dance, performances and exhibitions on contemporary art and culture at its venues the Royal Festival Hall,

Large 1960s development on the South Bank. Built largely of concrete, views on its architectural merits vary considerably. Presents a varied programme of music, literature, dance, performances and exhibitions on contemporary art and culture at its venues the Royal Festival Hall,
Check the official site before travelling; hours and prices can change.
More museums

Sir John Soane's Museum
Free, tiny, deranged — Soane's own house preserved exactly as he left it in 1837.

The Wallace Collection
Free national museum in a Manchester Square townhouse — armour, Boucher, the Laughing Cavalier.

The British Museum
Free, vast, civilisationally heavy — go for one room, not all of them.
Source: wikivoyage