Lie on the floor, lift the feet with partially bent knees and crunch the abs to bring the upper back off the floor in a hollow position. With a medicine ball or similar in one hand, perform a sit-up while bending and lifting the ball-side leg to create space to pass the ball under the knee to the other hand. Once the ball is passed, return to the original hollow position and repeat the sit-up and ball pass in the opposite direction.
Basketball abs trains both the abs and hip flexors, adds some unilateral hip flexor demand, some anti-rotational demand, some balance… and some fun and variety.
Sets of maximal (not actual maximal to failure) reps work well, which may be anything from 20-100. They can also be done for prescribed periods of time or in intervals rather than reps.
Pauses can be added at any point to increase isometric work, the ball can be reached farther out to the side before passing under the knee to increase anti-rotation and demand, and if flexible enough, the legs can be kept straight rather than bending the knee to pass the ball under.