Having an 11 yr. old and a 7 yr. old girl who are very different from one another, I'd have to say the only common threads would be sparkly things and lots of colors. Beyond that, Buttercream's suggestion about asking her parents what she's into in a really good one.
At 9, most kids really have opinions about what they like. Ex. my 11 year old loves blue, black, green, peace signs, skulls, doesn't like to have her hair done, hates dresses, and is into music and gymnastics, whereas my 7 year old loves pink, purple, flowers, princess things, loves dresses, having her hair done and is into dancing and animals. (11 yr old doesn't like animals at all!)
The whole peace sign thing is really big right now, both of my girls like stuff with them on it...glittery stuff is also very in with that age set right now too.