One of my fave ab moves is the side plank with an arm wrap. (petting the cat is optional)
STEP 1. Start in a side plank, legs stacked & hips high. Keep your elbow/shoulder/arm all in one straight line.
STEP 2. Lifting your hips higher, and wrap your arm thru. Return to Step 1. Repeat for 10-20 reps then switch sides.
I usually do sets of 20. Beginners should start off with sets of 10 each side.