In my opinion, re-using the art from old calenders is a great way to recycle, and put great cheap art on the walls- you can even customize it to the season. Especially in a dorm or a rental apartment, where it's against the rules to drill into the walls and hang things, it's fun to just cover the walls with posters and photos. My closet door features a photo collage of some of my favorite pictures, and it makes me happy whenever I see it.
In my room I also have some screen printed concert posters (which make a way better concert souvenir than a band t shirt):
As well as two Steve Keene paintings (and my Frida doll of course).