The player has 10 balls to serve; he places them next to the baseline and picks them up one by one.
At the end of the 10 serves, for every miss (hit the net or out of service box), he will have to perform a certain task (e.g. 5 push-ups, 20-second jumping jacks, etc.).
Notes:
To avoid “pushing” the balls in and stimulate players to add more power to their serves, a rule can be made: the second bounce (or third for younger players) should be over the baseline.