It took me some time to decide on a concept for the first round, but eventually I settled on this simple music box. It uses 100 elements, is complete on all sides, has an opening lid, and the crank turns the cylinder which in a real music box would in turn pluck different teeth of the steel comb to create the melody.
I also gave a try at using the model to make some "music". Something that didn't sound too difficult in theory, but I quickly realized I had no idea what I was doing...
Thanks to some help from John, did manage to get a short video put together. Which you can see with more images on Brickbuilt.