Here are some ideas of standing ab exercises that you can try today, just make sure you talk to your doctor first to make sure it's ok:
1. The first standing ab exercise you can try is to stand with your feet shoulder width apart, raise your arms straight above your head, slowly lower your arms while simultaneously raising one of your knees, keep your leg and arms at a 90 degree angle, put that leg down while raising your arms back up and alternate the same thing with the other leg. Do at least 30 sets on each side.
2. Stand with your legs shoulder width apart, take a step forward with one leg, carefully lift the leg that is forward a few inches off the floor, slowly make circles using just your foot - try to keep the leg still and straight, make 20 circles in one direction then (using the same foot) circle 20 times in the other direction, than alternate with the other leg.
3. Stand with your legs shoulder width apart, hold a 10 to 20 pound dumbbell in each hand, slowly lean to one side keeping your legs straight - this move should only involve your obliques (side abdominal) muscles, after you've completed 20 reps on one side, do the other side.
Talk to your doctor before you start any exercise plan and if possible find yourself a personal trainer so that they can watch you and make sure you are using the proper form, otherwise you may injure yourself and that will only slow down your weight loss efforts.