For tweens you will want plenty of big action. Take them to the mall and make a game from Christmas shopping. Break them into teams and give them a list of activities to accomplish and along with 5 bucks for the purchase of a gift. You can specify any number of crazy requirements for the gift. For example they must purchase a gift with the colors pink and orange and the letter g that is not made of plastic.
Points are awarded on a scale for how closely the gift specifications are met. Give each team a camera so they photograph themselves each time they accomplish one of the activities on their list, like: pose with Santa's elf, have the whole team share an orange Julius, or mimic a window display.
Other ideas for Tween parties: night at the movies, ice skating, and Christmas around the world.