Using balance cushion:
Rollup - works core
Lie on your back with the disc under your lower back, knees bent and feet flat on the floor. After placing your hands on the sides of your head, move your torso forward and squeeze your abs forcefully. Slowly lower yourself down and repeat. (3 sets of 10 reps)
Uneven squats - works the thighs and glutes
Place one foot on the disc, place your other foot slightly wider than shoulder length away. Slowly lower yourself down by bending your knees, stopping when your thighs ate parallel to the floor. Steadily rise back up, repeat for a set of reps and repeat with your other foot on the disc.
Using balance ball: Do 3 sets of 10 reps of each, these exercises work either core or glutes