1. There's a ton of free public domain art available online, like from public library sites or Wikimedia Commons.
2. Display keys from everywhere you've lived.
3. Order these incredible (fake) signed pictures of random celebrities.
4. Stack books to make a low surface for display.
5. Make garlands from paper cut-outs.
6. Use paint chips to cover an entire wall.
7. Download this adorable printable calendar.
8. And these even-more-adorable 3D letters.
9. And these patent-design prints.
Download them from Primer Magazine.
10. And these simple wall flowers.
11. If you want to decorate with wine/beer bottles or jars, nail polish remover will help take the labels right off.
12. Use newspaper to make a wall design, like a map.
13. Or just straight-up use maps.
14. Re-cover a boring or beat-up lampshade.
Check out the directions here.