Can you put up posters? I realise you have no budget, but it wouldn't be difficult to make some with basic musical elements, notation etc. I can help with ideas if you wish to do this. Is the room as tidy as it could be? Don't mean to be rude, but often these rooms can get messy because no-one feels that it is 'their' room and so take no responsibility. Does the piano get dusted regularly and are the keys clean? Is the room regularly cleaned by the cleaners, bin emptied etc? What colour are the walls? Would a spring clean make a difference, even if you can't paint them? Is there room on a wall for a whiteboard - perhaps there's one in the school that isn't being used. This would not only be a useful thing for the teachers, but would also reflect light. Have you asked the music teachers what they would like? Be aware - you may find that music teachers using the room have a legitimate cause for complaint if there is no opening window. I seemed to have asked a lot of questions, rather than given answers, but hopefully something for you to think about.