This exercise seems to be working great.
Lying on a mat, feet on floor with knees bent. Upper body should be raised in an isometric contraction with shoulders off the ground (as though you did a normal crunch and were holding it up). Put a dumbbell between your feet vertically so your feet are still close together and extend your legs out and pull them in toward your chest while maintaining your upper body position. I do sets of 25 reps and with a 5 pound weight it really works both legs and abs.
Hope you enjoy this one as much as I do!