Art is as art does. They tend to stick to strictly visual decorative arts.
Music boxes are a particularly good example of a non-magical treasure.
In our world, the cylinder units, because of the difficulty in changing the cylinders for new tunes, were replaced by the disk units very quickly, so there is little overlap between them. Both types were made in very elaborate cases, with lots of inlay and intaglio, fancy carvings, and so on, but collectors favor the quality of the music, which can be quite considerable. It is not unheardof, for example, to find a beat up old music box that has an original composition by Mozart pinned into its cylinder - one that's completely unknown in any other media. Such a box might have great significance in ancient times, perhaps playing a tune that had deep meaning to some great historical figure, the equivalent of Christ or Alexander the Great. Gears, levers, spring motors, and so on will not push up the tech level of the average campaign but will make possible new concepts of treasure.