hmmm, now to my mind, you buy a bookcase because you need to put something in it?

Geez I hope you didn't purchase a bookcase just so you could buy stuff to put into it. A bookcase store books usually. There are other items of furniture better designed to display 'stuff'. Some with glass shelves or lights to properly display stuff.

I would get rid of the bookcase before I was obligated to purchase more stuff so I could use it.

It would help if we saw a photo of the room and bookcase.

To fill it, I'd start by assembling books from all over your house/apartment and grouping them by height, color, and/or attractiveness of the bindings. I'd also look for any existing objects, such as....who knows what you have.....geodes, book height objets d'art, small framed etchings, book height baskets, pitchers, vases, or urns, carved wooden boxes, dried leaves, bowl with balls of yarn etc.

If you feel you're short in the objets department, I'd recommend looking at local thrift stores, antique shops, consignment shops, etc. (Avoiding major retailers unless there is a specific item you want/need.) The thrill and creativity of the hunt is a major part of the fun and you should only select an object if it's really meaningful to you, not to fill space. Consider managing the colors, textures, and shapes of the objects....don't let them be fully random. Include metal finishes if you can.

Consider cleanability/dustability as an important factor.

gd job qam, always someone takes this stuff for real.