Just from what I can see in the pic and if I had spare tiles I'd convert it to a cupboard, because you can nver have too much storage, and fix the floor tiling.

Otheriwse it depends on personal taste and whether you have anything to display. I mean a tall ornamental vase could work but not if that's not your thing. Or you can add a series of shallower triangular shelves and display smaller items but only if there are things you want to see. Another question I suppose it was does the corner face, it's juts a hallway so only seen when walking past, not something that needs to tie in with existing decor anywhere or that will be in line of sight when you are sitting somewhere?

Something like this, with artificial foliage if the light/conditions are not suitable for a living plant or you don't want the maintenance.

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Some storage if you don’t want a sculptural piece there https://www.shelterness.com/corner-storage-units-saving-space/pictures/78721/

Here is the image

Who uses hallway is it part of public space?

It's in my house, and it is the hall from the front to the back of the house.

I'd thought of a plant/planter but I think they would tend to overhang or protrude into the walkway which I don't want - it isn't very wide.

It is visible from the back room, I don't want storage there as it will just gather clutter.

I was thinking of a sculptural thingy there but can't find anything the right size.

You can get plants and pots in all different shapes and sizes and colours . Whats your favourite colour ? Everything seems off-white , but I'd even 'steal' the basic idea from Macyjeans pic -- get a nice piece of ply , cut to shape , do in a reddish stain and varnish if required -- it will make that corner warmer . Basically , have a wooden 'triangle' instead of a blink-and-you-miss-it ledge that blends with everything else ( and I mean everything ! ) .

Then add say a red or maroon pot , with a taller plant , say a lancewood or similar , even a cactus ( maybe not if you have children ! ) .

Another 'slight' possibility would be an artwork ( or two ) on the walls to lighten and brighten , but that may make the ledge look even stranger , although you could do say an art object or even 3 candles or vases on the ledge , in complementary colours to your art .

Nothing on sites like Etsy?

Garden decor places might have larger scale decorative items.

Make your own "sculptural thingy". See if local colleges or art schools run adult evening courses.

Incase I was not clear my suggestion was to convert to closed storage, and take the opportunity to fix the tiling, though an eye-catching display will distract from that.

You could have a beautiful ceramic pot with artificial fiddle wood plant.

You could have a beautiful textured rattan tall storage basket.

You could have a tall floor lamp.

,,

What a perfect spot, you could find a gorgeous pot, add magnolia branches, beautiful ceramics on a stand keep it simple and minimal for a stunning look.

A pendant light could work with a coloured or even mirror top on step