For my daughter I quilted the a flower stitch vertically. From far away the quilting almost blends in, but close up it’s a great detail.
Quilting knit turned out to be really easy! All you do is get some knit fusible interfacing and fuse it to your fabric. Then stitch or embroider your knit fabric with a ballpoint or jersey needle. I bet even fusible fleece would work too to add warmth to your knits.