Wall decals are a great way to change the tone of a vertical space without painting or hanging anything. They're easy to remove, too. These are all available on etsy. The one above can be bought here. These next two can both be found here and here, on Popwall's etsy store. They're clearly the same decal in different colors, but just that change, the placement, the color of the wall behind it, and the placement make a big change in how they look.

There's also a more whimsical choice.

Or something with more of an old fashioned feel.