Bicycles - I do 3x20 (10 on each side) for these. Keep your shoulders off of the ground and alternate and tap your left elbow to your right knee and vice versa.
Standard Ab Crunch - Squeeze your core and lift your shoulders off of the ground and contract your abs. Rest and do 3x20, or adjust based on your ability.
Alternating Oblique Taps - Lift your shoulders off of the ground and alternate back and forth touching your hands to the outside of your foot. Rest between sets & do enough to work both sides.
Planks - Hold plank position for 30seconds - 1 minute, making sure your body is parallel to the ground. Can be modified by resting your knees on the ground as well!