add wood wallpaper to the kick plates of the stairs. Those rolls are expensive but you would only need one. Not practical with white treads though, it will be stuffed up from black shoe soles in no time.
I plan on having lots of kids, at least so if space is limited, I just love the idea of having a loft bed built into the eaves of the house with two single beds below. It leaves a lot of floor space too, so no need for a toy-room.