I do a mixture of stability and flexion exercises to target the core. Primarily for stability I have clients perform a variety of russsian twist, planks, and antirotation exercises based on fitness level. Flexion exercises consist of stability ball crunches, V-ups, bicycle crunch, captain chair crunch, just to name a few. Depending on what type of equipment you have, you can have some fun with variations of medicine ball toss and TRX crunch/planks.
Hope this helps