Stability balls are great for core strength as well as balance and flexibility. You can use them alone or add weights to help build upper body strength. 

• Heavy-duty, 300 kg (600 lbs.) anti-burst rating
• Available in 55cm and 65cm diameter sizes
• Made of non-toxic vinyl
• 100% free from latex, lead and phthalates

Active sitting (for first time users): Stimulate vestibular system, improves balance; strengthens core muscles

Progression: Try lifting one leg and maintain balance and posture; Move ball in all directions using pelvis only, keep core engaged to stabilize torso

Exercise Instructions with Images: