Hi Allan,
You know from past posts that I use my physio ball regularly. It is a great way of reducing stress on the spine while strengthening the core. It is important that you seek professional guidance before you just grab a ball and start working out. Form and posture are crucial when using the balls. An easy core activity is just balancing on the ball. Use it as a chair. Use it to do the bridge pose, abdominal crunches, and reverse crunches. The ball can also be held in front of you while you squat- you should feel this in your glutes and quads. Hope this helps!