Here is a great kettlebell mobility combo I designed to stretch the back/spine. The supine spinal twist combined with a kettlebell flye. The great thing is that light load is added to the opposite side of where the legs turn to, this keeps the opposite side (loaded side) closer to the ground and gives a deeper stretch. While bringing the legs up and performing the flye you also have two opposing forces which is great for the core.
More details here.