Have you ever tried a wavy stitch? My machine can do it as a quilting stitch even with a walking foot. I'm thinking you could do a wavy stitch either straight lines or diagonal in one section, crosshatch in another section, and echo in another (like suggested around the hearts maybe). It would be cute to do them each differently.
You could google quilting with a walking foot and get lots of ideas and samples to pop up. I can do gentle curves and if you can do that you have more options. Whatever you do the quilting will make this quilt because you have so much open space. What a sweet thing for you for do your friend! And what a lucky baby! Please show us when you finish it ok?