My first thought was to do a vine like stitch in the sashing using a variegated light green thread. Then I would pick a small motif (flower or geometric shape) and do it in the center of each block. I would do this fairly small and either match the block for top thread or use invisible thread or just keep using the green variegated thread. (Use the green for the boobin throughout the quilting.) This would create a nice pattern on the back and would hold the top together without distracting from this so cute panel. Please let us see what you decide to do.