Boat pose is great for working your core abdominal muscles. From a sitting position, come into a V-shape by straightening your legs into a 45 degree angle. Point your feet. Tilt your torso back (also into a 45 degree angle), relax your shoulders and hold your hands above the floor. Ensure your stomach muscles are sucked in in the process. Keep breathing and maintain for 2-5 breaths, working up to 10+ breaths when you improve your strength.
Side plank pose gives your obliques, the muscles at the side of the stomach, a great work out. Starting in plank pose, stack your left foot on top of the right, maintaining a rigid core. Engage your legs, firmly press through the right hand and raise your left hand. Keep your hips high and pull your stomach in. Stay in the pose for a couple of breaths, building up to 5-10 breaths.