in cases like that, I often use the stability ball or a dyna disc. Here are examples of entry level core exercises with the ball:
Have the person sit and hold the ball on her lap. Ask her not to let you push the ball away from her as you apply pressure on the ball in different directions. You can start with predictable and increasing pressure and then move on to random movement. This is also a lot of fun.
With a dyna disc, I have the person sit on it on a bench or chair, being otherwise unsupported. Then ask the person to move the arms from side to side to challenge the center of gravity. Eventually, you can add weight to that, a weighted ball, for example, or tubing.