I like to use the stability ball for variations, I always find new ways to keep challenging my abdominals.
I also use lots of cable exercises, hanging exercises, floor exercises, bench, etc...
Rep range varies a lot, sometimes 10,12,15 reps sometimes 25 to 50 reps.
I can do a circuit of exercises with 3 to 4 different variations, 10 reps of each from 4 to 8 sets! Depends on how I feel. My goal during training is the quality of the movement followed with the mental connection between the muscle that is working. I am a big believer in quality rather than quantity.
An example workout is as follows: