If you stand in almost a squatting position and then rotate your pelvis forward, this will stretch everything in your lower back that usually gets tight after sitting a while or exercise. Make sure when you pull that pelvis in, that you’re holding for a few seconds. That makes a difference!
Downward dog into cobra pose is very good for stretching the lower back. Also being on all fours and doing happy car sad cat where you extend your back up and down is beneficial for your mid back. Finally lying on your back and trying to touch your knee to cross the other side is a great stretch.