A large piece of art as wide as the doorway.

It's hard to tell from the photo what your design style is. The choices are limitless really. A grouping of any type of collection, pottery, plants, etc. I personally find it more pleasing to the eye to have a combination of items placed on the shelf and then hanging above. Whatever you decide on, keep the triangular shape of the niche in your design. Would love to see pics when you're done!

HI -- Try this -- lots of things , some tasteful and some not . Designtoscana.com -- something -one thing ,long and flat . Maybe some angel wings , it depends on how deep the ledge is but I like the shy angel with the long wings . Anyway I think you will find something you like there . You could also mount a big round ceiling medallion there or something big and interesting like big old signage letters . Also look at the décor items at Restorationhardware.com -- everything there is tasteful . Just remember to go big and one or few (3) and no faux plants they are just dust collectors and only for show homes .

Ignore the shelf part.

Go to a local architectural salvage store and buy a decorative piece, maybe wrought-iron scrollwork. Hang it directly on the wall.

A plant or artwork.