Compete in stuff, win it and you get trophies for free
Seriously though, pictures of members having a good time are great. Cheap to get printed and frames are fairly cheap.
Our house has themes on walls. In our main room, we just have lithographs of the major paintings of the fraternity, plaques that show awards like "Brother of the Year" and gifts from other organizations.
Upstairs we have a lot of autographed pictures of famous alums.