Side bends: just stand straight with feet apart. Put your hands on your pelvis and flex laterally to the right, back to the starting position and then bend your left. This exercise is great for the obliques.
Push Ups or lizards: besides being a great exercise for the arms, are very good for toning the abdomen, to make this exercise simply place your palms on the floor and put your feet together.
Try to keep your whole body right and bend your arms getting into the stomach to make your muscles work too abdomen.