If you care about your couch at all, you'll buy one of these Mario or Luigi hat cushions from NCSX. It's the least you can do, considering what you do to it all day. You know, the sitting.
Alternately, you could buy one of these plush toys for decoration. That would be pretty weird, though, right?
Also in today's NCSX update: awesome Bomberman t-shirts and cell phone straps, Chocobo cushions, and even more Animal Crossing toys.