Cheap ways to do up my room?

i live in the attic and it's freaking huge! people expect it to look awesome but are always disappointed because i dont have it decorated. the living room part is white and all it has in it is a led zeppelin poster (black red and white), a pink shag rug, a pink lava lamp, a purple couch, and in the other room is my "bedroom" which is green. the carpeting is light brown. there is a tiny kitchen area which has LEgo death star model on the table for decoration, i have a mini fridge, and a tv and a few garfield comics on the walls. i would like to find something to do with the 8 bare walls including the landing, stairways, and actual walls in the room (besides putting up a million posters, i dont want it to look like juno's locker), please help me decorate but i only have about 50 bucks to spend!! thanks!!fyi i dont have curtains. are there cheap alternatives?