How should I decorate my room?

I have a four wall room. One wall has a balcony and closet so that doesn't really count. I have a bunk bed, and I can't really put anything on the walls cause I have wallpaper. My door has a collage on the front and postcards on the back. I have a keyboard and two guitars. Oh, and my closet has 3 posters on it. I have a study table which has a small picture of Mona Lisa. My walls are lavender. I think my room is sort of boring. Any idea on how to decorate my room?