Ideas for goody bags at a 13-year-old's birthday party include planting kits with pots and seeds, cookie cutters, colorful flip flops and mugs filled with the makings for hot chocolate. Activity kits are also popular ideas; try giving a friendship bracelet kit, or make your own kit with everything needed for a manicure and pedicure.
By the time a child is a "tween" or teen, she is less likely to want a goody bag filled with candy and disposable trinkets. Plan your goody bag around one key item that a 13-year-old girl is likely to appreciate after she gets home from the party, and consider tying the party favor to the theme of the party.
For instance, after a cooking party, send 13-year-olds home with a cookie cutter or two tied to a few favorite cookie recipe card. End a pool party by handing out fun flip-flops, or just leave them in a basket by the front door for kids to grab on their way out.
If the party involves any crafts, use the final product as the party favor. For a slumber party, have the guests paint their own pillowcases with fabric paint, let them dry overnight, then send them home as the goody bag treat.