You can relax your back and open your hips with Happy Baby Pose (ananada balasana).
• Lie on your back and bend your knees into your chest as you exhale.
• Spread your knees and feet as wide as you can, as though you are bringing your knees into your armpits. (As in the photo in the link above.)
• Reach between your knees to grasp your inner arches (or toes or ankles) with your hands.
• Pull gently with your arms to set your hip joints and open your hips as you lengthen your back.
• Breathe easily.
This is a great yoga pose to incorporate undulations, as shown in this video.