There's a nice circuit in the 2nd Edition that goes like this:
Sit ups (if weighted, hold plate behind your head) 10 reps
Hanging leg raise 10 reps
Ab Wheel 15 reps
Side bends 20 per side
You can do this after each 5/3/1 workout, or fit it into your schedule how you see fit, just get it done 2+ times a week.
Repeat circuit 2 times the first 2 weeks
Repeat circuit 3 times the second 2 weeks
Repeat circuit 4 times from then on.
Progress by adding weight/reps/circuits, move on from knees to feet in the ab wheel, etc...
This circuit hits everything if performed correctly - do the reps slowly, fully contract the abs.. really the basics. I've never seen any need to change my core routine from basic sit ups, leg raises, ab wheel and side bends. My core is relatively strong and I have no back issues at all - but when I do core exercises I don't flail around like a dumbass - I brace my abs and make every rep as hard as possible, no momentum. Stop between every leg raise, pauses a the top of every sit up, control the weight during side bends. Again, the basics.
Side bends are awesome if you do them correctly. Lower the weight slowly, explode up, unilaterally loaded. Focus on keeping all of the core musculature tight. Again, the basics to any ab routine.