Fitness ball exercise

One that I hada lot of luckwith was a sit up on the ball. Sit on the ball, roll forward to where you are laying on the ball and do sit ups from that position. Untill you are pretty steady on it I would suggest staying close to something. I always did mine in the living room near the couch. Now bedroom near the bed..

How about laying on your back with your feet up on the ball, lift your hips up into a bridge, then roll the ball in toward your body & back out again. It does have some knee bend but you're not actually putting pressure on your knees.

Also just a standard plank but with your hands or forearms on the fitness ball. Focus on keeping your butt down.

Lie flat on your back with the stability ball between your hands up behind your head. With your arms & legs straight lift both into the air to transfer the fitness ball from between your hands to between your feet & lower arms & legs back flat to the ground. Lift arms & legs again to transfer ball back to between your hands. Repeat. Doesn't sound very hard but it's effective.

Planks on the swiss ball
Russian twists on the swiss with a medicine ball or some form of weight
Umm.. I can't think of anything else.

Good luck with the back. It's a real pain when you get an injury.