I bought a second bobbin case for my machine so I could set it for "bobbin stitching" and marked it with a dab of red nail polish to set it apart from my regular sewing bobbin case.
The adjustment screw on bobbin-case is only good for so many adjustments, then you need a new bobbin case anyway.
You can do some really nice decorative stuff with the heavier threads, even some lightweight yarns if you use them in the bobbin, threads that would never go through the eye of a needle.
You can do this with a zig-zag stiich as well as a straight stitch.
I have a good book on this somewhere. If anyone wants, I will look up the title and author.
You need to keep in mind the decorative stitching is going to be on the bottom side of what you are stitching on. So have the back of the piece looking at you under the needle so the decorative stitches will be on the "right" side when you are finished.
Joyce