Wracking your brain for an idea of what to get a 12 year old? Me too! I started seeing her come home with rubber band bracelets (think those little round ones you find in with hair clips, brushes, etc.). Here's a link to one you might find at Walmart (though my Walmart had a cheaper price than online for the one they had on display): http://www.walmart.com/ip/Wonder-Loom-Rubber-Band-Bracelet-Kit/30652494
I also spied a loom at AC Moore and they actually had theirs limit 1, so you could only buy one loom at a time. They had a spectacular amount of different colored bands too, which you'll want to stock up on.
If you wanted to make some yourself without a loom, I think you could do it with just a crochet hook and some patience. The rubber bands sold in concert with the kits at the stores I have seen them at also included the clips, which you'll need to secure the rubber bands in a bracelet shape.