The British Library (no, not the British Museum, althought it's great too). The British Library has a small museum inside, it only takes about two hours to see everything, but inch for inch, I think it's the best museum in the world.
Everything from some of the first maps of Europe and America, Gospels that were left out of the Bible, John Lennon's handwritten "Yesterday", the first sketches of Winnie the Pooh, a Magna Carta, a Gutenberg Bible, . . . all in a room about the size of a basketball court.