If you want a challenge with your drawings, try human faces that are upside down or at an odd angle. It forces you to measure and makes it hard to use some of the crutches that you may have unknowingly developed. There are also subtle distortions that I notice consistently reappearing in my drawings. I've always wondered if that had some relation to the dimensions of my face. Maybe you've heard the Leonardo/Mona Lisa speculation. I've learned a periodic return to more precise measuring helps me recognize these flaws and adjust accordingly.