Ah, the absolutely unusable ledge.....
I might suggest a collection of oversized baskets - grouped together in a nice vignette. Gives you texturewhich doesn't show the dust - because any plant you put up there will obviously be a fake. (Who can actually water it?) Pottery Barn is all about baskets this spring.... I like the idea, frankly, because they are space fillers without being too cloyingly "decorative". Just be sure they are lovely, large and simple. No bread baskets or easter baskets, please....

Would love to see what you ended up with.

Still never finished this area. Any other ideas?

How about a huge artificial fern sitting up there? At Christmas I would have a tree there with large boxes wrapped beautifully.