You've gotten some good ideas, but if you are not comfortable with free-motion quilting, or want a more modern feel, try parallel straight lines the length of your runner. I recently finished this quilt for a customer and I love how it turned out. She asked for the lines to be 1/2" apart, but you wouldn't need to do them that closely. I'd probably do them so they're bisecting each of the little squares in the four-patches - it looks like you'd need about 11 lines?
Whatever you decide, it's going to be adorable - I love the bright spring colors!
Andi