I started piecing my backs a few years ago and now I do it on all my quilts. I think that they add personality to the quilt plus, they help eliminate leftovers in the scrap bin.
My last baby quilt I made I used leftover blocks pieced together and then used my leftover fabric to make large strips around the center block. The back was cutier than the top (in my opinion). Anyway just let your creativity rule. You'll be surprised what you end up with.
The only thing I would suggest is not to try to have anything that has be to centered or symmetric. It can be done but it can make it a little difficult.