Knit about 20 rows of garter stitch, picking up a different strand of yarn for each row. Leave a tail at the beginning and end of each row. Because you are using a circular needle and leaving the yarn-end tails, you don't have to turn the work around to go back. Pick up a new strand of yarn and start across again. Garter stitch borders of this width are personal preferences.
Seed stitch works fine, as well.
10 ridges on the end balances with 10 garter stiches on each side and so on. As before, when you work each row, pick up a different strand of yarn, leaving a tail at each end. This method of working without turning is especially convenient when the Afghan gets large and heavy towards the end. By varying your choice of yarns you can get stripes, random colors, color families, etc. It is amazing how yarns you might not think would work together do in a project like this.