Personally, I favor Seated Waist Twists as active rest between a Tri-Set of Abdominal exercises. Here's an example: Cable Crunches, Hanging Hip Flexion, V-Sit-Ups. Then, Seated Waist Twists for say 1 minute, before performing all over again.
All of the aforementioned exercises can be found within our Exercise Index.
For further Ab workout ideas, visit our sample workouts, see our gym workout books, or download our gym workout app (it's free!). All exercise demonstration images are from the GP Shuffle workout app.