Players work in pairs with a ball between them. They underhand pass the ball to each other but while standing on one foot only. Players can move but can only do this by jumping on one foot, with the other in the air.
To keep the players imbalanced, tell them to underhand pass the ball while standing sideways.
This exercise works the calf muscles and improves balance.
New hitters rarely miss because of their arm. They miss because of their feet. Build a reliable approach, then time it with your setter.
When your hitter gets stuffed, five teammates standing and watching is how you lose the point. Good attack coverage turns a blocked ball into a second swing.
Coaches spend hours correcting arms in serve receive when the real fault is almost always the feet. Fix the movement and the platform fixes itself.