The Side Plank is a great core exercise to target your abs, but especially your sides. It also has lots of variations so can really challenge your core strength.
Works: Core, Transverse Abdominus, Rectus Abdominus, Obliques.
Start lying on one side, feet on top of each other,then lift up onto one elbow. Push your hips high into the air and hold for 20 seconds. Keep your legs straight and hips stacked on top of each other, then repeat on the other side.
To challenge yourself more, you can balance on your hand rather than your elbow as shown in the photo, or try to dip your hips down to the floor then lift back up again ten times.