gentle core work-
For me the big inflatable exercise ball seems to work and be do able.

I just sit/balance on it and move hips side to side , back and forth- side bends /stretch .
I tried bouncing on it but feels like it is kind of jamming my spine a bit so i don't do that anymore.
But that is on the DVD of exercises that came with the ball, there is a lot more that I hadn't even thought of but some are just using the ball with lift and stretching.

Oh there is one where you lay on floor and grip ball between feet and use core /legs to lift ball then do side to side with it.

I use it for stretching by laying over it on my stomach and my back and rolling it slowly to and fro.

since it is a round ball the core is used to help stabilize you anytime you are on it.

also don't forget isometrics - focus on just tightening muscles and hold 6 sec release and repeat - way better for us than trying crunches or sit ups!