Although we need rest, it's still important to remain active on those days. The body, like the mind, needs stimulation every day, even after a grueling marathon most people still take a walk, as early as the day after to avoid stiffening up. Just remember on rest and recovery days it is important to avoid doing the worst thing you can do for your body...nothing.
A stability ball is a great addition to standard workouts as they help core muscles to activate. Core is what supports your posture and daily functions, but often overlooked and underutilized while working out.