If your client is able to do them, I am a big fan of stability exercises such as the Plank for core. There are numerous ways to progress Planks (on the ball is just one way to progress) and they are much more beneficial than sit ups (repeated full flexion can do more harm than good for the lumbar region). If your client cannot get up and down from the floor easily, then exercises with cables or weights such as wood chops or standing torso rotations are good–it really just depends on your client’s limitations. I am also a big fan of using the TRX for core stabilization exercises. The TRX can be used for almost every fitness level.
I hope these suggestions help you.