I stretch in the morning but if I was doing something before bed I would do a forward bend.
Reach up high with both of your arms, feel your back stretch. Reach up a little buggy on your right and then your left. Now take a deep breath and slowly bend/fold forward while you breathe out. Slide your hands down your legs as you bend forward. When you are fully bent over you can wrap them around the back of your legs. Let your body hang and breathe slowly as you feel the stretch and relaxation. Then very slowly bend back up again. When you are straight up reach your hands over your head once more and stretch upwards one more time. Take a deep breath, slowly release and you are done.