Unfortunately, nearly every exercise you can think of will engage your abs. That's because they're your stabilizing muscles; Even something like a squat or and arm-based upper body work out will engage your abs in some way!
While you're recovering from your surgery, you really need to abide by doctor's orders, and get a physical therapy referral if you can manage one. That way they can design a program based on your doctor's guidance. The odds are you're not even cleared to workout yet! Focus on healing, and you can recover strength later.
Weight loss is about the food you eat, not the exercises you do anyway. You need to talk to your doctor about what is appropriate for you; since you're recovering, it may be wiser to simply set yourself to maintenance and focus on healing, first! You have the rest of your life to lose weight. Better to focus on healing now, rather than risk injury or slow your recovery thanks to impatience!