I love piecing the backing also ..... here's my favorite look .... but as others have said.... there are many different ideas.
I pieced a row of 10" squares together and used them as the vertical center of the back... adding enough fabric to either side to complete the backing. On another quilt I sewed all of my leftover jellyroll strips together and used it as the center strip.... again adding solid fabric to either side. I sure hope this sounds as simple as it actually is. If not ... please give me a holler and I'll try again lol ......