A stability ball is a fun toy to have around no matter where you work out. By toy I mean just that: throw it, bounce it, use it for a ground and pound speed bag, handball, etc. But the portable, relatively cheap stability ball, also known as a swiss exercise ball or just exercise ball, is also a serious muscle-building, strength enhancing gym tool. How? Just add dumbbells!
There are a lot of free weight exercises that have no place being executed on stability balls, but combine swiss ball exercises like the hip bridge with dumbbell flyes or presses, and you have an excellent core workout that makes this traditional weightlifting movement more challenging. In fact, just about every exercise done on a swiss or stability ball requires serious core activation, which means more muscle and strength, for you.
You can read all about the best free weight exercises to do on that big inflatable ball here:
Best Free Weight Workouts on a Stability Ball
Learn which swiss ball to use and choose from the best brands here:
Swiss Balls, Stability Balls, Exercise Balls
Great reference book!