Here's a lesson plan to focus on the 2nd most important shot of the game: the return of serve.
I used my Ropezone to divide the court diagonally from ad net to deuce baseline. All players are on opposite baseline. I fed a simulated serve into the deuce service box. We then played 21: players are trying to be the first to get at least 21 points. They get 2 points for returns hit cross court and 1 point for hitting down the line. If they are hitting a lot of net balls, I may choose to deduct one point for any net balls. Anyone landing on 13 points had to go back to zero. First player to 21 wins. Then move Ropezone to opposite corners, move players to ad side and repeat.
Now that the players have warmed up their returns, let them warm up their serves. Commence time round round robin, singles or doubles. Score game as usual EXCEPT any missed return of serve is GAME OVER. Keep track of games won per player. Player with most wins at end of round robin wins. You could also keep track of how many players lost a game by missing their return. Player with fewest misses wins.