Heres a workout thats uses supersets to hit all areas of your stomach, Your upper and lower abs and also your internal and external obliques.

Superset
Hanging leg raise – 10-20 reps x3
Rope crunch – 10-20 reps x3

Superset
medicine ball sit ups – 10-20 reps x3
laying scissors – 15-20 reps x3

Superset
oblique teapot – 10-20 reps each side x3
laying side crunch – 10-20 reps each side x3