Well aside from money (which you can never go wrong with) what does she like?? You could always pay for a trip.. like my grandmother took me to Canada as a gift.. now that's a bit pricey but what about a hot air balloon trip, or a cruise. You could also resort to music or clothes?!
The gift should be something that represents the birthday girl. Ideas, thank you note wrapped around here favorite candy. Her picture on a small cake box with chocolates. Cupcake wrapped in cellophane, with a ribbon that notes the date and name of the quincernera. It should be unique to each girl. Oreo cookies dipped in chocolate wrapped festively.