We did our daughters room for her 9th birthday. I let her pick the paint colors (from the bedding and wall border she choose) and I added gold metallic paint to a creamy white and rolled that on the top half of the wall, added a cute border, then sponged the bottom half of the wall with 2 tones of pink paint. I stayed away from themes because we were on a budget and did not want to re-do the room when she was no longer interested in whatever theme we used.
There are a lot of ideas we found in the Jcpenny catalog, they call them tween rooms I believe. My mom bought her the bedding and the wall border so we were only out for paint and supplies. I know Wal-mart has some very cute bedding and wall border for not a whole lot of money, you might check there.