We put a tall, dried flower arrangement in ours

Multiples of candle sticks with candles, multiples of glass jars in colors and heights, a very large Chinese vase; Google on Houzz, what to put in a niche on a staircase, or you can sheet rock over it and solve the problem with nothing on it.

A small picture.

Add an easel to sit the framed picture on.

Do you mean the entire space or the niche only?

If just the niche, something in the same shape, like this terrarium from Terrain.

If the entire space, we need another photo.

Art pottery.

Your house’s millwork reminds me of the 1920s and thus the niche reminds me pf a phone niche. I would put an antique phone in it.

We love art niches and have several. Think about an appropriately sized art piece OR a nice vase with or without flowers.

I would put a clock or a plant that can take low light in it.

I would put a potted plant with trailing leaves - pothos? spider plant? Both do pretty well in low light and don't need a lot of watering. Maybe an orchid?

A feathery fern - maidenhair

Anything you would enjoy looking at often. And maybe reminder notes & shopping lists for on the way out.

Stack of colorful classically bound books on their side is another option.

Paint the of inside of this niche a slightly darker shade, then add a sculpture or vase that you love. Dried flowers - not so much - in time they always get dusty. If you want a plant, use a fresh orchid plant, and replace when it has faded. Start with looking through the collectibles you already have. The piece must be nearly as tall as the niche. In my house I would put a tall blue and white Chinese jar.

Because it appears to be on an outside wall, I would use a museum wax to stabilize the item that you display there. Sometimes when closing the door or running up or down the steps you jiggle the item, you don't want it to fall.

I agree with Dorothy Meyers. If you put anything that is breakable, whether on an interior or exterior wall, use Museum Wax.

Amazon.com: Quakehold! 66111 Museum Wax, Clear 2 Ounce : Arts, Crafts & Sewing

We put a carved wood tiki trophy and some seashells in ours. We live at the beach and it works for our fun casual style.

Other favorite idea is to fill a clear glass urn/vase with blown glass globes

A tiny original oil painting would be adorable in an ornate frame that doesn't compete with the trim work in your home. This space is lovely and doesn't need much ornamentation like statues, imo.

A glass urn with seasonal items such as Christmas ornaments, painted wooden Easter eggs, a bird's nest with wooden eggs, acorns/fall leaves....

Maybe black iron candle holders, any thing mate black low profile vase with small greenery or even a twig. Simple and organic/ raw metal, wood, etc…

Keep it simple. If painted, a soft hint of the color of the door handle, keeping with the colot of the stairs. Then, a tall orchid. BEAUTIFUL!

Seems the area needs a bit of color. I'd do a floral arrangement, in a pretty vase, changed out for the seasons

A sculpture of The Blessed Mary

Not meaning to be morbid, but do you have an attractive urn of a loved one’s ashes? It’s a sweet memory.

A vintage clock

Flowers/faux or dried in a vase. Adds a certain happiness and warmth

I piece of ceramic pottery or art glass

Make it go bye bye. Patch with drywall.

Something you love. There is a suggestion in FengShui leave an area of wall, decorative space empty
 This let's you know in the future you will find something to place there.
For now a vase with bright colors works.
And remember there is something in the future.

How about jazzing it up with some tile!

I would put 3 candlesticks of varying heights (wood would look nice) with antique-looking battery-operated candles. Get the kind that uses AA batteries and has a timer so they come on each evening.

Invest in a stunning crystal/ mineral specimen, geode, natural history etc.

A lamp with a timer attached, that is artistic capturing nature.Like the one depicted here.

World Market or other places like flea markets, etc.

At Ethan Allen we have a variety of beautiful faux florals. If you would like to e-mail me with the dimensions, I would be happy provide you with some options - emily.forster@ethanallen.com

Have a great day!