Make sure your room is reasonably tidy, and clean. You want girls to feel like they could get naked and not worry about where they sit, lol. Lighting matters a lot - make sure you have some lights that give a good feel in the evening. Beyond that, whatever works for your interests. It doesn't have to be what interests her, as long as its relaxing and clean.
I think coins idea is good and intresting you can also hang waste cd's on your wall with a thread like a wall hanging it looks good .make that room an exhibition of yoursef which you would like to show to others.your intrests likes and dislike and other things
i really recommend going to marshalls, tjmaxx or ross, bed bath and beyond also, they have amazing decor and cool stuff, I got all my room stuff from there and its so cheap too! marshalls is the best!
Add atmospheric lights! I have blue light coming from underneath my bed and behind my couch. I also have lights which react to music. People like it and I totally love it myself, I'm really into the spectrum lounge concept :)