Core workouts

Howdy folks
I need some suggestions on ab works to do on my off lifting days. I haven't really focused much on my core and need to start doing so on my off days.

Any suggestions? 

I bought a $10 ab ball from Walmart, pretty much all exercises on the ball are core work, the poster that comes with it instructs how to do all the exercises. Love it

my core workout typically looks like:

6 sets on the ab oblique machine (3 sets for each side)
4 sets on the ab pulldown marchine
3 sets on the back machine
4 sets of crunches (2 regular, 2 oblique)
3 sets of reverse situps
3 sets of decline situps
3 sets of leg pullups

Planks and i find that my heavy squat day really works my core harder than direct ab work.

P90X ab ripper X
P90X+ core workout
Power 90 Ab Ripper 200

All good core workouts.

Kim