wacky. do you have anything to display, like sculpture or vases? If so, I'd fill these with plywood and then top that with gravel, and use the spaces like display ledges, for art, flowers, or whatever. It's hard to tell how much they intrude on your stairwell, or I'd be tempted to have them removed, but that could get tricky...

Do you have any interest in continuing the plant theme? Something like golden pothos is extremely easy to care for and will improve your indoor air quality. Spider plants and Mother-in-law's tongue are also easy and great for the air. If this isn't your cup of tea, then I agree with the first comment; display area of some sort.

Cover them. Display stuff. Easier to dust and keep clean. I would top with book shelves and create a little soft seat some where along there.