The 13 Best Abs Exercises You Can Do Standing Up

Standing in a low lunge (left leg bent at the knee, right leg straight behind), extend your arms over your head.
Bend slightly forward so that your weight is transferred to your left thigh.
Bring your right knee to your chest using your core muscles and lower your arms to your sides.
Bring your leg back to the floor as quickly as possible and then repeat again.
Do 20 reps on each side.

To complicate things: Add a twist, trying to touch the left elbow to the right knee.