Audio geeking.
Many audiophiles will spend a ton of money on electronics and cables and other gear and stick it all in a room that sounds like shit. Room treatments are some of the best bang for your audio buck... if you can work corrugated foam into your decorating plans.
I just found a company that makes acoustic panels covered in fabric for as little as $22. With mounting hardware, so you can just hang them on your wall like a picture. Lots of different colors, including some patterns. Sonically they basically work like an open window, making your space sound larger. Offer some interesting visual possibilities as well.
I'm ordering two of the coffee sack panels for my guy room.