I try to do exercises with E on our large, bouncy ball a few times a week. I have her sit, lay on her back, or lay on her tummy. We started doing these exercises when E was a few months old at the suggestion of my sister who is a pediatric physical therapist. I always have my hands on her, and I keep the ball steady by bracing it between my knees. A great book that encourages this type of activity with babies is Active Baby, Healthy Brain.