You can't paint the walls, but covering them is definitely recommended. For goodness sake, make sure you don't use contraband hangers. This means no nails, no screws, no super glue. Ask your resident assistant if you don't know what is OK to use. Mom and Dad will not be happy paying for wall repairs at the end of the semester.
Posters are an ever-popular option.
There's always the standard Bob Marley or naked girl. Or hang license plates in honor of your new friends' hometowns. Get some canvas and use acrylic or spray paint to make a custom painting. Another option is to try theme decorating. Make your own Cajun bungalow with Spanish moss, taxidermied gators and fishing tackle.