The internals of a PC hard drive (and maybe an optical as well). There are various rotational speeds, it would be cool to see a 5400rpm, 7200rpm, 10k, and 15k drive compared next to each other showing the read/write heads flying around, etc.

Opticals get spinning fairly fast and have a much larger medium inside them, but the laser head doesn't move around frantically the way a hard disk does...