I need help decorating a teenagers bedroom without spending any money?

im 13 and my bedroom is really boring. im not allowed to paint the walls because we rent a house! i have a dog that is always messing with things that r hanging from the ceiling! my mom is cool with hanging things up on the walls. i have a lot of shelves in my room and not a lot of books so i have a bunch of nic naks on them. i also have 3 windows and a window seat. PLEASE HELP, IN DESPERATE NEED!!!!!!!!