Here is the first from a series of five that I will be doing:
As you can see, it's a rather simple design. Just a basic spiral. It's a little hard to add much more to something this small, but I tried to do something fun with the ending ramp. The path dips sharply coming out of the spiral, and by the time the marble reaches the end it has built up a good bit of speed. It reaches the upright portion and tries to climb right up it, runs out of steam, and falls back on itself where it comes to rest in the bottom of the "U."
Above is a fun view of things and kind of gives you an idea of the roller coaster feel it has when it finishes.
For a simple piece I think it turned out pretty well. After finishing it I immediately started on another one. I hope to have it completed by the end of the week!