Here is a beautiful core strengthening sequence to inject into a vinyasa flow or do on its own.
2. Take your right leg to the sky for three legged dog.
3. Roll your weight forwards, round your upper back and bring your right knee to touch your nose and back to three legged dog.
4. Bring your right knee across the body to touch your left elbow and take your leg back to the sky.
5. Touch your right knee to your right elbow. Bend both elbows down into chataranga and perhaps tip your weight forwards so your back leg lifts.
6. Repeat with the left leg.
For increased challenge, hold each step for 3-5 breaths. I sometimes like to add kapalabati or fire breathing to this sequence if I am feeling energetic.