I think I would try to have "functional" items on "my" shelves - decorative, but useful.
Because - in my space, I have a lot of "stuff" - and "useful" is more important to me than "just to look at"
Do you have "others" that come into your space? Kids or pets? If these items are within reach of pets or kids, they should be non-breakable and non-injurious. The only things I can think of off-hand are large empty spools in plastic containers.
If you have lots of space and the chances of the items getting knocked off are small, then whatever suits your fancy.