Maybe decide on a theme for the miniature collection to be placed in those boxes, such as gemstone rocks, birds, shells, figurines, artifacts that go with the art you have above, etc. Or something you'd like to make for it.

A small collection of colorful glass pieces, or carved stone, interesting wood... A three tier diorama of a colosseum or life of commoners to dieties- I'm into miniatures so I see little scenes in there, heh. You could do artificial plants instead of live ones, or get artwork in outdoor materials that look like plants. Like brass, copper, glass..

The two commentators have good suggestions.
The current configuration on that wall makes a "face" ...two eyes,light fixture nose and semi grimacing teeth.
You may consider changing the position of the "eyes".
You may also try to fill each of the box spaces full enough to minimize dust collection.

That appears to be outside. Was there a purpose to that grate in the wall?

Agree with shells and rocks

Sea glass, look on Etsy under: raw sea glass

Maybe just leave them empty and I agree the 2 pieces that look like eyes need to be changed .