Today I discovered the magic word: Skeleton. A "skeleton" watch is one where it is purposely made to be see-through. "Full skeleton" means that even things that you'd kind of like to have on a watch, like numbers and hands, are as tiny and hard to use as possible so you can see the full glory of the gears.
Feast your image display software on THIS:
I gather it is possible to spend up to and including ONE MILLION DOLLARS on a watch like this, but, I further gather, Chinese imports can be had for under $100. Even well under $100. For instance.
I wouldn't have many requirements on such a gift. It should be as visible as possible and be actually mechanical, no hidden battery doing the work.