Build a complete core like a gymnast

This exercise is not usual and it is terrific for building a core like a gymnast.
It could be too difficult for most people but you can challenge yourself and give it a try.

Lie on your back and raise slightly both upper body and legs: your entire body has to be in full muscular tension, tense hard your abs and push your low back against the floor. Hold the position for 5 seconds, take small breath in and out.

Then, roll sideway to put yourself on the stomach, do not touch the floor with the legs and upper body at anytime, keep your entire body under tension.
Hold again 5 seconds and get back to the starting position.

Do a few reps, as far as you can maintain a perfect body alignment. It is a complete core exercise I learned in gymnastics when I was a kid. You work especially your abs and low back, but all the muscles work in synergy!

Check these additional resources for abs workout and back workout.