Just had to write about one of my most favorite pieces of equipment: the stability ball (aka the physio ball).
There are a VARIETY of exercises that can be done using the stability ball to work out your ENTIRE body. And the investment cost can run you as low as $10 for the ball!
(And $3 per hand weight if you don't have any.)
For the beginners
Sit on the ball & "walk your feet" infront to where your knees are 90 degrees, your back is straight, and your toosh is half on the ball. Once form is correct try these exercises: bicep curls, tricep overhead extension, overhead press, chest fly, and chest press.
Lay on the ground, knees bent, with feet on the ball. Try to strech out your legs while moving the ball forward (legs still on top) and then move the ball by bending your knees in towards your body. (Targets lower abdominals & works legs). Leave legs on the ball & try to to complete a crunch with your legs at 90 degrees. This targets your upper abdominals; throw in a twist to target your obliques.
For the more advanced:
Balance on one leg, the other leg have bent with your shin resting on the ball. Try bicep curls, tricep overhead extensions, chest fly, chest press, and overhead shoulder press.
To target the abdominals: assume plank position, but rather than having your toes on the ground, stabilize & put them on the ball. Try to hold for 15 to 30 seconds then break & bring knees to ground. To build on this, try bending your knees while in plank & bringing the ball in towards your body, then stretching your knees back out to roll it away.
To target legs even more, try a lunge with your back leg on the ball as opposed to your foot being on the ground for stability.
Something that I have yet to master, but continually trying to achieve is balancing on the ball with my knees bent & no part of my body touching a wall or the ground. Goals are good!