It’s very difficult to provide one stretch for back-pain relief, as everyone is so different! But what nearly everyone needs is to improve motion. Try folding forward over your slightly-bent knees while clasping your hands on your opposite elbows. Just let your upper body weight hang nice and easily over your knees, getting your head as close to the floor as you can without straining.
The second one involves a foam roller. Everyone should have a foam roller! Place it on the floor, perpendicular to your spine. Lie with it under your back and roll up and down the roller as high and as low as you can. Try to bend your back around the roller and stretch your back along its length.
With both of these exercises, deep, smooth, full breaths the entire time will make a tremendous difference! Your spine should be moving in a wave with each inhale and exhale all day long, but especially when you’re on your foam roller!