The cow / cat combination helps my lower back. I also like the stretch where you are on your hands and knees and you extend one arm and the opposite leg and hold, then do the other side. And, of course, child’s pose. These all help.
This is one given to me by a physical therapist. Lie on your back. Keep your shoulders flat to the floor. Lift and bend one leg up so it's at a 90 degree angle. Your calf should be parallel to the floor. Gently move your knee over the opposite hip. You should feel it in your hips and lower back. Bring the raised knee as close to the floor as you can without forcing it. Hold for a ten count. Gently bring your leg black up and straighten it so it's flat on the floor again. Switch sides. Repeat a couple of times for each side.