I never start with static stretches for personal training sessions but some dynamic warm up. I found people are so ready for training after them.
Walking lunge (deep) and Hip lift
Start with walking a long stride into a deep lunge position to increase the adductor stretch, keep the back flat but chest up. In the lunge position, try to squeeze the glute muscles and drive the hip toward the ground to increase the hip stretch. From this position, keeping the hands on the floor then extend the knee to increase the hamstring stretch on the front leg. And do the other side as well. By extending the knee, your back may be round but should be okay.
Walking lunge (deep) with one arm up
Going a long stride into a deep lunge position just like the previous one, then get one arm up to the sky. This way, it adds the thoracic spine movement. Make sure to maintain the low back flat throughout the movement.