Tone my Stomach!!

I know that losing my stomach starts in the kitchen but for the other 20% is exercise what are some good workouts to help firm my stomach. I'm not looking to get a six pack but just to get a flatter stomach than what i have now. I eat very lean protein like salmon, chicken, ground turkey and eat lots of veggies, my carbs consist of brown rice and potatoes. Any suggestions would help. Thanks for reading