There's lots of stuff out there, even with the WP Marketplace being "small", so knowing a bit more about about this particular girl's interests could probably help. Like examples of any games on consoles she's been into recently, and does she mostly listen to music or has she been playing with music-making stuff as well?
I don't have kids myself, but I'm pretty sure that it's an age where you don't want end up suggesting things that she finds childish. And I do remember being 12 myself, after all...