planks really are effective when it comes to ab workouts.
Do pull ups, if you have that luxury, they work everything you want
Try hip-ups, mountain climbers, and planks. Try doing crunches on a stability ball. I've found some great workouts with the stability ball at ab-workouts-guide/stability ball. They have low impact on your back. Also try 2 Min Full Plank Challenge, Multi-Plank Challenge, and Ball Blaster.