The National Gallery houses one of the finest collections of Western European paintings in the world — over 2,300 works spanning from 1250 to 1900, displayed in a purpose-built neoclassical building facing Trafalgar Square. The permanent collection is entirely free.
On any given visit you will find Botticelli, Caravaggio, Velázquez, Rembrandt, Turner, Constable, and Van Gogh's Sunflowers. The Sainsbury Wing, added in 1991, holds the earliest paintings including the Arnolfini Portrait by Jan van Eyck. Don't miss the Leonardo cartoon — a full-size preparatory drawing for a painting that was never made, now one of the most studied works in the building.
Friday evening late opening makes this a good choice for avoiding the midday tourist crush.
Nearest tube: Charing Cross or Leicester Square.