One house had these three reliefs in plaster. The first one and the third one are depictions of Johannes Gutenberg, inventor of the printing press. I do not know why, I have found no association between him and Idstein.
Some houses had some sort of coat of arms (more like coat of tools) carved into the woodwork. And one house had a cat climbing the wall which was unfortunately too high up for my puny phone camera to take a good look at.
The last atypical ornament was this faun, looking mournfully over a garden fence. I have no idea whether it is a modern addition or genuine antique, but it fitted the town nicely and did not stand out as inappropriate. Which does not prove anything.