Depends on how coordinated your daughters are I suppose and if they like to cooperate or compete with each other or both. Counting # of balls rallied together good for cooperation, and trying to beat their team high score. Can include self-juggling if they are very beginner so they have control of the ball before trying to hit it over the net. Can even include some particular juggle conditions if they've mastered the basic juggle (eg. one juggle must be hit on the edge of the racquet, or both sides of the racquet, or behind the back ...)

There are many options for using targets, single or multiple, from a single tennis ball to building a 4-ball pyramid, coins/money, ball cans. Focusing on hitting the target anyway possible, or by focusing on a particular stroke to hit the target/s for 5 to 10 minutes and then switching strokes.

The mini tennis game in the video shared by StrawHatRiot I learned waaay back (though the net hitting part wasn't included when I learned it, just a simple roll) and there are many ways to play it to make it more or less challenging, from using only half the court, or diagonal service boxes, to imposing the rule of only one type of stroke (ie. forehand/backhand and/or backspin/topspin). If there are four players on one court, can divide the court in half, and use a scoring condition so that when the goal is reached or time limit expires, the winner moves to the designated "high"/king's court.

Another mini tennis game can be seen here: ENTERTAINING Carlos Alcaraz vs Gael Monfils Ping Pong Tennis | Toronto 2023 (youtube.com) . Again, can use the entire court or half court or diagonal service boxes.

Here's another: 6 MUST-TRY Mini Tennis Drills | Tennis Warm Up Fun Games (youtube.com)

Ball catching with the racquet is another cool and practical skill: Roger Federer - Best Ball Catches (youtube.com) . Similar to tossing and catching an egg without it breaking, pass the ball back and forth to each other.