Here are a variety of stability ball core exercises that you can do.
Just make sure that you can master a plank before you move up to these more advanced progressions.
The core exercise progressions are as follows:
1. Elevated Plank (e.g. elbows on bench)
2. Floor Plank
3. Stability Ball Rollout on Knees
4. Stability Ball Plank
5. Stability Ball Rollout (Knees Off Ground)
6. Stability Ball Stir the Pot