The mass of the stuffed monkey is less than the mass of a person, so the stuffed monkey will accelerate more and the person with more mass will accelerate less. This is because the heavier object has more resistance due to it's mass, so it doesn't want to move as much.
As the tennis ball hits the ground it causes force on the ground. Creating an action that is equal and opposite to the reaction, when the force of the floor reacts when the tennis ball bounces back up.