I made this quilt, too, but in a wall hanging size. I stitched in the ditch and then echo quilted around each shoe because I have very little imagination and just wanted it finished (better finished than perfect). If you want to do a little more to it, but are intimidated by the wonderful suggestions by the really creative people here, just quilt the background area in a diagonal pattern.
Your flip flops are really cute! I also embellished mine with buttons-it was fun hunting for cute ones that coordinated with each pair of shoes.