Maybe unorthodox, but do deadlifts maybe? I don't know if that'd hurt less than crunches and whatnot, probably would. I know now I can do way more situps than before I started deadlifts and clean and jerks. What deadlifts do is they isometrically contract the abdominal muscles, to keep everything from not collapsing, and yeah... Lamar Gant is a famous powerlifter, he was 130 pounds or so, but he had a 600+ pound deadlift, but one thing about him was, he actually had scoliosis. Be careful with the deadlifts, as you can still hurt your back, but I'd say overall they're a lot less risky than squats. Worst case scenario in a deadlift usually, the bar just stays on the ground... Cleans and snatches, the Olympic lifts, are good for core strengthening, too, but I don't know if those are a bit much also.