He discovered the Moon’s cratered surface, the moons of Jupiter, and the phases of Venus, proving that Venus orbits the Sun. Galileo described the motion of falling and accelerating bodies, formulated the law of inertia, and correctly predicted that, in a vacuum, all bodies will fall at the same rate.
One of the most famous legends about Galileo describes how he dropped cannonballs of different masses from the Tower of Pisa. You can explore this experiment with the Free Fall Tower Gizmo.
In the Gizmo, you and your students can drop a variety of objects from a tower, with or without air. You can even deploy parachutes to further emphasize the effects of air resistance! This Gizmo provides an excellent introduction to free fall and acceleration.