My nieces are 14 and 15. They love anything from Cotton On or Forever New. Handbag is a good option if they don't have one, or a new purse - just make sure it's 'cool' and not one for a nanna! Itunes gift cards are always an good standby if they have an ISomething. Magazine subscription? New bathers? How much do you want to spend? What about a voucher for a local hairdresser to go and have a cut and blow dry? Most teens love a bit of pampering!