I just took a machine quilting class and she does stitch in the ditch around everything including borders and then does fill in the differents areas. For example in your inner squares you might put a star or some other meaning full object then quilt feathers or leaves or a continuous pattern aroung the next square. You could do some really fun interesting stuff. Check out the quilting books around to get ideas.
Some day I want to do a square in a square pattern.
I just saw the straight lines in your post. You can do X in the center of the small sqare and grids in the outer areas aroung the square.