either the 'out' signal
or reminding us she's
Two players face each other across the net. One is server. Server has a small hopper or other supply of balls, 20-30 max. Non-serving player is across net with an empty container. They must retrieve the served balls and put them into the empty container. They DO NOT have to retrieve any ball that does not land in the correct service box. Ideally they will watch the lines carefully and call 'out' and give the proper hand signal (index finger in air) when serves are out.
Continue until all balls have been served; switch roles and repeat. Repeat entire cycle on other half of court. Or, move server to other side of court (not end) halfway through the process.
If server serves into the net or whiffs, he/she may retrieve that ball and try again.
You may add a competitive element by counting how many of the balls end up in the retrieving player's hopper (indicating how many of total balls have been hit into correct service box).