What is the easiest and useful move for stretching the back?

Most core street he's also stretch the back as it is part of the core, extending the abdominals constricts the back and vice versa, so torso twist, expanding the chest and pulling your arms back, then pull your arms forwards and constrict the chest, rolling your body down towards your toes slowly and slowly rolling back up, things like that

Counter Stretch – lean forward placing hands on a counter (like you would find in a kitchen or anything of similar height), and bend from your hips. Your entire upper body remains parallel to the floor. Hold for 30 seconds. I also like Downward Dog, Cat-Cow, and Puppy Pose which I believe are all standard yoga poses. A foam roller is wonderful for releasing tension in the back, however you should roll your upper and lower back separately to avoid pain or injury.

Stretching is a powerful exercise to release tension from the mind , one of the key areas is the back! You can try these moves :
Downward fog
Bridge
Touch your toes
Reach you arms up
Circle your upper waist
Those are the ones I can think of right now but I will let you know if I find any more effective, the two I would strongly recommend, as I do stretching, yoga and meditation daily,is the bridge and downward dog , hold each pose for minimum 1 minute , thanks for asking this question please ask again if you have any trouble with anything! Be fabulous