Wondering why the cabinet doesn't go to the top of the niche? It looks odd. It really isn't a built in, but a complete cabinet inset and then the baseboard meeting it. Don't faint away on me, but would you ever consider removing them? Then you could have chests, lamps, or art, etc that would be so much easier to decorate than buying all those smalls to fill the shelves.

You could repurpose them:

One initial thought is that they are pretty dark - it would be fun to mount wallpaper against the backwall - you can adhere it to foam board so it's not permanent - even something simple like a neutral grass cloth could help them come to life

- there are lots of good articles out there on how to style shelving - I think being patient and not buying everything at once at a big box store can help the items be more unique and personal (love vintage and antique stores for cool finds)

- don't be afraid to include bigger items, including art keep it from looking too cluttery - repetition of items (white matte ceramics or brass candlesticks) can help bring a sense of curated style

- family photos can look great, I would recommend consistent & simple frames with photos matted to give breathing space - simple frames enable the photo to be the focus

have fun!

some helpful articles:

https://www.thespruce.com/styling-tips-for-open-shelving-1975823

https://lindyegalloway.com/styling-open-shelves/

https://www.marthastewart.com/7617497/how-style-bookcase

Removing is a big hassle. Will have to work with it as is for now

I do have the option of moving the shelves, so I can increase the space between them and have 2 instead of 3

Do you collect anything?? You can personalize your space by showcasing a “collection”. I am a retired US Air Force Captain. In my barrister bookcase shelves, I have some of my military memorabilia; my collection of miniature porcelain English buildings (collected when I was stationed in England); my collection of midcentury glass condiment containers (some were my grandmothers); and my collection of porcelain open hands. When I visit friends and family, I love seeing their collections displayed.

Lots of helpful tips, but the important thing is that it appeals to YOU.

I need to see the whole room and this is IMO one more reason to not have display shelves that had no purpose to build .

I don't think removing them is a big hassle. There is just the small piece of molding on either side that you remove and the cabinets will pull right out.

Otherwise, because they don't fit the niche, you could paint them the same as the walls. Doing that might disguise the gap at the top.

I agree with @judianna20. Very odd and awkward that they weren’t built to the ceiling. Because of that, I’d paint them the wall color if you are dead set against tearing them out.

WAIT and plan to redo the entire fireplace wall. The hearth is too high, the surround too short, and TV should be somewhere else. There are lots of ways to improve the overall vignette here, but they will be pricey. For now, you need 1/3 hardcover books that you can even pick up second hand. They can stand, lean, or stack lying down to support some other decor. The decor items should be large enough to be seen and identified from the seating. If you want more interest, wallpaper or paint custom cut pieces of luan or foam core to slip into the back of each shelf.

I don't know....books? and some collectibles that have meaning to you (which could include photos in pretty frames). I don't think they look odd at all...not everything needs to be "magazine perfect".

"The decor items should be large enough to be seen and identified from the seating."

Another good tip!

If you go onto Pinterest there are helpful articles and lots of examples. This is a job that requires a lot of patience and comes down to trial and error. The basics are height and visual weight, vertical and horizontal and most important they have a vibe and some empty space is a good thing.

https://lcliving.ca/tips-for-styling-shelves-and-built-ins/