To make the abdominal exercises more fun, I sometimes do the following exercise.
Have two players sit on the ground at about 2 meters from each other and let them pass the ball overhead.
Every time they play the ball they have to move their backs back towards the ground and come back up with their abdominal muscles when the ball is played to them again.
This will not always be a perfect game, but that is a good thing
If they have to play the ball next to them, they will also immediately use the oblique abdominal muscles
Make sure that you have some balls in your hand to give to them immediately when the ball rolls away, otherwise the intensity is too low.