Back: Cat, cow, child's pose sequence
There are many, many muscle groups that span the length and breadth of your back. And because these muscles are involved in so many different movements at so many different joints, it's hard to choose just one stretch that does a good job at maintaining mobility and easing tension.
That's why I've chosen a common yoga-based sequence that really does a good job of targeting your entire back, while also stretching the chest, shoulders, abs, and hips. Plus, the cat-cow exercise is known to help alleviate low-back pain, an all-too-common plague of modern life.