I learned these [tag]indoor softball drills[/tag] in college, but have used them with high school and 12-U teams.

4 Corner Drill
Divide evenly into 4 corners of the gym (for description purposes, N,S,E,W).   Start with 1 ball.   N throws ground ball to S, S throws to W, W throws to E, E throws to N. Rotate clockwise after throw.

Add another ball – start opposite position (N to S & W to E), then another.   Mastery of this [tag]softball drill[/tag] will yield 4 balls going at once.   This is a great drill to end your [tag]softball practice[/tag].   (Players must rotate around to starting position twice).

Safety Considerations:   I suggest Incrediballs at first. Make sure rotation occurs around the drill. Do not allow the S line to “cheat up” for the ground ball (watch W to E throw)

Drill emphasis:   concentration, accurate throws, footwork, communication (target hands/verbal cue for   player receiving next throw is essential with multiple balls), conditioning.*
(*We sprint to next position, and stop to run when a ball is thrown away.   After sprints, everyone goes back to their original position.)