I can only tell you how I would do it. I would stitch in the ditch in the blocks. You can try to do that by free motion but if you don't have the confidence yet, just use your walking foot. I would find a simple quilting template that would fit in the square and quilt that design in each of the full squares. You can find a stencil if you wanted to put something in the sashing but it is difficult to find something with the correct length that will match at the intersections. You could mark a simple crosshachting design to fit in the sashing or just leave it blank. It is all up to you and what you feel comfortable doing. Your selection of fabrics was excellent. Nicely balance layout.