Here is a somewhat random object: The model of a molar tooth in paper.
I made this primarily to test my workflow for creating semi-realistic models with a low enough number of polygons so that they actually can be built.
Still, even with the simplified form, this model is a bit difficult to make. This is partially due to the complex template, but also owed to the fact that the model has deep grooves and saddle points which make it a bit difficult to close the shape.
Nevertheless, if you want to try your hand at the molar, here is the PDF template: