You could use random squares separated by a good basic wide sashing. I have come to like pale gray...it helps colors pop, doesn't get dirty as quick as white, isn't too dark, is gender neutral.
Another option is to embroider some squares with some easy redwork pattern or some such to fit in with a limited number of squares of fabric. Like if you have transportation fabric, have a few blocks with stitched wheels, or bones with dog fabric, etc.
Just a few thoughts!