Recently I've been digging through my old note books of fitness routines and exercises, and came across an oldie but a goodie; Lower Ab Leg Raises with a Stability Ball.
This is one of my favourite lower ab exercises because it is effective and works! Every time I do this exercise, I can feel every muscle in my abs the next day.
Just like you see in the picture above, you lie on your back with your core tightened and a stability ball squeezed between your ankles.
Then simply raise and lower the ball slowly (with control) from 90 degrees to the floor and back again. That's one rep.
I make this a little harder by starting with my legs lowered to the ground, then lifting my legs and arms at the same time to the middle of my body. Then I transfer the ball from my ankles to my hands and lower the ball to the ground behind my head, and repeat (see the picture below).
Try this exercise out and you will see what I mean; you will feel it the next day!