If you don't have a stability ball just yet, you can use a pillow or do all of this without the ball. It's still a great workout!
Planks - shifts, leg lifts, shoulder taps
Ball below the taps of the shoulder blades - crunches, pulses, table leg crunches, add twist,
All fours - 90 degree leg lifts, pulses, hug in, combo
Fire hydrants, pulses