"What did Leonardo da Vinci invent?" Perhaps a better question would be "what didn't Leonardo da Vinci invent?" da Vinci's ideas were far beyond his day, and while many of his sketched prototypes never materialized, his sketches include sound aerodynamic and mechanical principles.
As inventors and scientists in later centuries rediscovered Leonardo da Vinci's inventions, they have attempted to create his inventions based on his detailed sketches. Some of the inventions, like the helicopter, have resulted in failure, but others, such as his hang glider, would actually have worked. This list should give you an idea of da Vinci's imagination:
Flying Machines: Leonardo da Vinci was fascinated by flying and birds, and he developed several sketches for machines that could enable man to fly. He sketched a detailed human-powered helicopter and a hang glider, among others.
Solar Power: When da Vinci worked at the Vatican, he designed a system of harnessing the sun's light using mirrors to heat water.
Military Engineering: When da Vinci worked as a military engineer, he came up with several ideas for defensive and offensive weapons. Among his ideas were armored tanks, moveable barriers, parachutes, machine guns, catapults and defensible bridges.
Musical Instrument: Reputed to be a musician in addition to his other talents, da Vinci created a bowed keyboard called a viola organista.
Drilling Machines: Prototype sketches for drilling machines, both horizontal and vertical, are found among da Vinci's engineering sketches. He initially designed a machine to bore holes in wood.
Calculator: Da Vinci is credited with the idea for the first calculator, which was a cog-wheeled machine that could perform rudimentary calculations.
Submarine: Years ahead of its time, da Vinci's prototype sketch of a submarine was underdeveloped and not very detailed, but it captured the possibility of a manmade machine that could travel completely underwater.
Single Span Bridge: As part of his efforts in civil engineering, da Vinci designed a single span bridge that would connect over an inlet.
Canal System: Da Vinci designed an elaborate canal system for the city of Milan, linking it to the sea, and modern analysis shows that the designs were completely workable.