Empty Bucket is a simple warm-up perfect for large groups of any ability.
Place a bucket or hopper in the middle of the court, filled with tennis balls. Designate one player to stand near the bucket. This is the Bucket Boss. All other players are scattered around the court. Bucket Boss empties bucket one ball at a time. Other players replace balls into bucket, again, one ball at a time. Goal is to not allow the Bucket Boss to empty the bucket.
Play for a designated amount of time, then switch Bosses.
Adapted from ThePhysicalEducator.com