What are some good exercises that will flatten and tone my stomach?

Cardio Mix. You can not target the abdomen specifically for weight loss - if you have adipose tissue that you'd like to lose, then consider an increase in cardio and a balanced diet. However, do not always do the same exercise or you will increase your risks for stress injuries. Mix things up with running, biking, swimming, or circuit training. Ultimately you need to burn more than you consume.
Generalized weight. loss. For flatter abdomen eat healthy & get physically active. Calories burned > calories consumed. To tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on back. Engage in regular aerobic exercise (i.e. running, swimming, Nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.)