Introducing wall stickers, sometimes called vinyl lettering or vinyl stickers. They are a budget-friendly way to add art and design to your walls. They can be removed without hurting your walls so they are ideal for apartment dwellers. To find them, I like to search Etsy. Just type in "wall stickers".
Here are a few:
Note, above, the Russian nesting dolls are wall stickers. How clever.
What can you do with wall stickers?