I love the balloon idea. I've seen similar added to borders and they started at the top left and came down about half way or a little more. Then the right side started in the middle or up a bit and came down to the bottom. I also would maybe make 3 balloons 2 on one side of the string and 1 on the other. kind of like flowers growing only balloons. If you machine applique the balloons you could also machine sew the string. This a very cute quilt and doesn't need much on the border.