Want to make sure that you hit every aspect of your core in one section of your workout? Do a tabata exercise, alternating between holding plank and holding the superman. Both your abs and your back will be worked, and both exercises can be manipulated with to be more challenging, or simple a little different to prevent boredom in your workout.
You will hold the plank position for 20 seconds, then rest for 10 seconds. Hold the superman position for 20 seconds, then rest for 10 seconds. This is one round. Do a total of 4 rounds, the entire exercise therefore lasting 4 minutes.
Make sure you give your back and abs a good stretch!!! Blast that core!!
Here's a picture of your standard plank. Be sure to have your elbows directly under your shoulders to avoid unnecessary strain on your shoulder and elbow joints. Push through your heels and extend through your neck, as if you are being pulled in opposite directions. Want to make it a little harder? Place your toes on a box to elevate your legs.
Here's a picture of the standard superman. This will really work your lower back, and you want to engage your glutes as well to keep your thighs off the ground. Extend your arms straight ahead of you, keeping your palms facing each other and your gaze at a point about 45 degrees ahead of you to keep your neck relaxed. You can make the superman harder by holding dumbbells in your hands, you can do superman pulses, or you can flutter your hands and feet.
So, play around with the different variations of the exercises and alternate them in a tabata-style exercise.