I think it is hard to know what to do for that age. Maybe you could do a purse cake with some bright colors. I also think the flip flop cakes seen on this site are really cute. You could do a small whimsical cake and decorate the tiers with her favorite colors. How about a present cake??? That is all I can think of right now!
I made a back to school cake for my 12 year old daughter with her favorite group on it, an edible image.....she and her friends freaked out....they thought that was so cool, they all just started 7th grade as well.....maybe you could find out her favorite music group and do a edible image.
The cell phone is a GREAT idea! No matter what she is into (girlie clothes, sports, etc.) what 12-year old girl is NOT also into talking on the phone! I believe that talking on the phone may be what 12-year olds do best, because it's what they do the MOST! Haha