The Play Like a Champion sign is a must have! Snow Bowl pic would also be high on the list.
My wife & I have also turned our finished basement into an ND room, in addition to several framed posters (4 to be exact) adorning the walls, I have several unique items: a framed & signed SI Cover of Mo Stovall catching a TD pass from Holiday vs MSU in Ty's first year, and two signed footballs - one by the 1998 starting offense & defense (I was a student manager), and the other signed by the 2003 seniors (obtained at the football banquent). IMO, unique items (read: autographed) can really make the room special. Other "filler" includes ND hats, a huge fleece blanket hanging over the couch, and countless other random stuff from the bookstore.
The encased brick mentioned above is also very cool!