A Swiss ball, aka therapy ball, stability ball, or exercise ball, can be very beneficial in strengthening the core. Make sure you feel safe and stable on the swiss ball, or have someone hold it for you.
The first swiss ball exercises are the pelvic tilt, lateral tilt, and hip circles. These are great for using only your pelvic and core muscles. Try not to move your upper body, and move the swiss ball just with your hips.
Then next two exercises are moving your feet with a hip flexion and long arc quad (LAQ). These are slightly harder because lifting your leg makes a smaller base of support, and it’s harder to stabilize the ball.
The final two exercises are adding your arms by lifting the opposite arm with your leg, and then the same side. This adds upper body movement to the already unstable exercise.