on my ab day i like to hold a dumbbell over my head and flex my abs.
i also like to do rotational ab work while on the ground going from one side to another. thomas delauer has some good videos and i've learned some good tips from him
This is the template I recommend for ab training: super set 1 : 3-4 sets of lower ab exercise x 20 reps super set with upper ab exercise x 20 reps super set 2: 3-4 sets of lower ab exercise x 20 reps super set with upper ab exercise x 20 reps 3-4 sets of an oblique exercise like decline bench romanian twists - 20 reps per side 1 set to failure of a functional ab exercise like plank or ab roller