One of my favorite core exercises for training adults and athletes is the stability ball stir the pot. Most people make the mistake of doing a ton sit ups, leg raises, and side bends. The abs are meant to create stability of the spine and help transfer force during dynamic activity.
For this exercise, assume a front plank position with your arms on the ball and your feet out wider than normal to create a stable base. When performing this, draw small clockwise circles with the ball, as you maintain a flat back and squared up hips. Repeat on the other side once completed.
This may look easy, but if you have never done this before, I guarantee you your abs will be feeling this one!