after traveling for years with my pre-teen and teens, nyc is a bit of a challange. try shopping at H&M for everything. Also try Loehmann's and Century 21 for discounts not found elsewhere and very good value. Teen girls love shopping and these will yield lots for little money. For window shopping try Scoop in the meat packing district.
For food, don't forget the frozen hot chocolate at Seredipity and candy galore at Lauren. While you're there, go to Bloomies to look before you buy at Loehmann's