You can do a circuit workout on a balance ball to whip your muscles into shape---a lot faster and easier than you could do it at the gym.
Balance balls come in small, medium and large sizes. It's a good idea to have two; a medium and a large. The two sizes will allow you to vary the intensity of each exercise, giving you more of a workout.
Part of the value of working out on a balance ball is its instability. Because it wants to roll, you have to use your core muscles to stabilize the ball as you use it. In fact, regardless of the exercise you are doing, you will also be working your core.
Be sure to do each exercise with proper form. You will know you are doing it correctly if you can feel the strain in the targeted muscle or muscle group. Each description below is one rep. Four sets of 20 reps is a good start point.
Start by working your arms. Place the ball centered under your shoulders, with your hands against the ball at shoulder height. Push up until your arms are straight, then slowly let your body back down onto the ball. Next, brace the ball firmly against a wall. Put your hands behind you on the ball, feet out in front, butt in front of the ball. Lower your body as close to the floor as you can, then raise it up. This will work your entire arms, plus chest and upper back.
For shoulders and abs, place the ball under your lower stomach and hip area, with hands on floor in front of the ball. Now, "walk" with your hands, letting the ball roll down to your knees; then "walk" backwards with your hands until the ball is back in the starting position.
Create buns of rock by laying on your back the floor, heels on top of the ball, glutes against it. Lift your body until it is straight, squeeze the glutes tight, then let yourself back down. This also works the hamstrings, the back of the thighs.
To work the whole thigh, lower abs and groin muscles, place your hands on the ball in the pushup position, with legs at a 45 degree angle straight out behind you, with the ball braced in a corner. Lift the knee of one leg towards the chest as far as you can; then stretch the leg back and up as high as possible before bringing it back down to the floor. Repeat each rep quickly before performing the exercise with the other leg.
Thigh, glute and core muscles can be worked with a balance squat, and you will get excellent core results by not bracing the ball against anything as you do it. Place one lower leg on top of the ball, move the other foot out two or three feet in front of the ball. Slowly bend the standing leg as far as you can without losing your balance, then slowly stand back up. Do all your reps on one leg before switching to the other.
One of the best passive exercises you can do on the balance ball is to use it as a chair. The slight core motions used to keep the ball still will give you a stronger mid-section along with greatly improved balancing reactions.