If you want a general guideline, but this is by no means a hard rule, walking foot for straight lines, & darning for curves or free-motion.
As for your quilting options, you could have a lot of fun with this project. You might want to run a ditch line between the sections for stabalization, but after that the options abound. For example, with your finger, trace around your clouds then trail on out into the sky area and "trace" another one free-hand... then sew what you just traced with your finger, On the grass section, you could sew up & down in a zig-zag, and around your geese as you did the clouds. You could also do an all over meander, stipple, or loops. And, yes, squiggly lines will look good as well. Allow yourself to have fun with it; it will show in your quilt. :D