Something that won't hurt the baby when he becomes more mobile and inevitably knocks it down! We have a dried plant/straw thing that isn't in a planter, but just kind of tied at the bottom that looks nice and is light enough that if our son knocks it down, it doesn't hurt.
Maybe a corner table or shelf--something 3-sided that will be stable. Since it's by the stairs I'd be forever knocking into any corners that jutted out.