Have you thought about wallpaper or fabric panels? You can be creative by covering old pictures frames, bulletin boards or even large pieces of cardboard. If you have worn out furniture that you like, take the upholstery off the back before you throw them out and use it for a fabric panel. Even leather will work. This will help soundproof too. You can even create a headboard with several panels.

Target has some cool wall decals that are pretty large. They are about $20-$25 for a roll that's about 3'x20'. The colors are very contemporary as well as the patterns. 2 rolls could cover a wall or so and they come off super easy without any residue.