When you live in a house so packed that you can not even buy another knick knack, you freak out over your walls. If the walls in question are really hard to drill or even drive a nail, you try to find happiness in wallpapers or murals and if watercolor murals can’t make you happy I don’t know what else can.
You can get the green mural above from anewall or various kinds of colorful stuff (and cheaper, yay!) from photowall. If you are braver you can try wallpapers like these brushstrokes from Emma Hayes and for s/he who has the biggest balls, try to paint it. Somehow. Throw paint cans on the walls or something.