Here's a personal favorite of mine for standing abs workout
FAST FLAT STOMACH WORKOUT standing abs - YouTube
And as others have said, cardio exercise to lose overall body fat will help get your tummy area back in shape. I know many have suggested walking, but I think more vigorous exercise will help you lose the body fat quicker. If impact workouts don't work for you, the elliptical is a great alternative. I can burn the same amount of calories on the elliptical as I can running...according to my heart rate monitor. Whatever you chose to get your sweat on, challenge yourself!
Best of luck!