Depends on their interest. If they are girly girls try jewelery, makeup, clothes, or even shoes.If they are both then anything.If they are a tom boy then anything sports like or interest them into something you like.Sometimes you can ask them ahead of time and see what they are interested in. That is what I would do.
Its is depending on their interest,some girls like sentiment gift like memory,letters ,scrap ..some likes useful gifts for use and need,like accessories,,stationary,dress..some likes costly gifts to show off like white gold chain,mac,foreign trip..I gave my 16 year girl for her 16th birthday,,a tea cup with her beautiful photo and written on it"sweet sixteen".my girl really loved it.she is sentiment type.
I'd say something like a golden bracelet or silver earrings. If you don't have that kinda money just give her like a trip to a zoo (or something like that) with her friends with food and all included..