Leg lifts.
Crunches/Situps
Twists
Mountain Climbers
Jogging (tighten core)

Anything cardiovascular really.

If you're wanting your abs to SHOW. It's one of the most complex muscle groups to be shown. You don't necessarily need trkout to get abs (you see all these skinny ass bitches with them) the layer of fat over your abdominal area is what's blocking them to be shown. Just simply eat better and you'll get greater results than just exercising.