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.
Can she plank and/or side plank? There are a lot of great core exercises for all levels of exerciser from a prone position.
Plank (lots of variation)
Bear crawls, or alternating hand, hand, foot, foot lifts while stabilizing spine
From standing, warding drills with the ball (i.e. trainer holds the ball, client presses the ball from varying positions) are great.
If you have bands, you can attach a band approx chest high, position the client 90 degrees to the band and have them do transverse rotation. Lots of ways to play with that – anchor height, foot position, direction of pull, etc.
every standing exercise that pushes or pulls against resistance is a core exercise. A standing row or chest press with tubing turns into core and balance when you have the client stand on one foot and/or have the exercise performed unilaterally.
I am a big fan of integrating core exercises into other movements like wood chops and such because that’s what the core does: stabilize during movement.
In addition to warding drills with the ball that Nancy suggested, you can do similar drills with a band: Attach the band, keep it tight and have the client hold the band standing sideways to the attachment, the client either keeps the upper body still while moving the lower body, or the lower body still while moving the upper body.
Example of what Janet and I described. I described doing this with the trainer holding a ball and the client holding pressure into the ball. What Janet would do could be the same foot pattern but putting tension on a band instead of a ball. Either works.
I filmed this variation (with permission) at a training with Michol Dalcourt talking about loaded movement training. A more common warding pattern would be to push the upper body in the frontal plane while lunging the legs back in the sagittal plane. This variation with the force and the movement both in the frontal plane is more challenging than it looks! I also like any opportunity for partner work that I can include in a training session.