Be seated, back straight and have your legs slanted out of the sides of the chair. Flex your abdominal muscles. Lift up your hands & arch your back, inhale as you do that.
Exhale & bring your arms down exactly in front of you. Repeat the process. This works your abs in a major way.
Plank Knee Cross
Place both forearms on the chair & step back in plank position. Tighten your abdominal muscles. Bring your right knee inside of your left arm, pause, return and then do it with your left knee & right arm. This works your abs, back & butt.