We took the little 'un to see the Leonardo da Vinci exhibition that's currently running in Exeter. The museum has done an admirable job in securing a loan of the sketches from the Royal collection, and I felt privileged to be able to examine them at such close quarters.
I was particularly taken with "A study of a woman's hands" (possibly for the portrait of Cecilia Gallerani), c.1490. Quite apart from its possible significance as a study for such a well-known piece, it contained, almost as a doodle, a lovely sketch of a grotesque head in its top left corner.
I also loved "A sheet of pictographs", drawn over an architectural plan, c.1490, also known as rebuses or cryptograms. A detailed description of this sheet is given in Charles Nicholl's biography Leonardo Da Vinci: The Flights of the Mind.