There are a numerous core exercise movements that you can do to really strengthen this area of the body. It is important to add strength to these muscles because they will virtually support your body throughout all the rest of your lifts. When you are not strong here, you will not be able to lift as much weight on the other movements you do therefore will not see near the physical gains you would otherwise.
The best way to make core exercise challenging is to use as small of a base of support as possible. This will then affect your agility and because these muscles are the ones that play the largest role when it comes to maintaining stability, they will be worked to the greatest extent.
You can do this by performing the core exercise on a ball or a wobble board. Simply standing and trying to balance on one foot while on one of these wobble boards (a board attached by a half ball) will really force these muscles to work. While it may seem like a movement that more focuses on the lower body, it definitely will work your abdominals as well. Just try it and you will soon find this out.
Also consider doing movements that have you extending and flexing at the hips. An example of this could be something such as a ball roll out where you start in a position with knees bent to the chest, balancing on the ball and then proceeding to drive the legs outward while moving along the ball. Once they are fully extended then slowly bring them back into the body once again to complete one rep.
Remember to keep altering the movements you do slightly so you are consistently working the muscles from a variety of angles to ensure that all the fibers are being worked. Doing so will help you make the most out of your abdominal and back sessions.