As the other have told you, the best buys are on silver and turquoise. You can get some decent deals on loose gemstones also. Be aware that there are street hawkers all over the place who will try to intice you into their stores, they are very persuasive, just ignore them, don't even make eye contact, just walk past.
You also must buy Mexican vanilla if you do any baking or know of anyone who likes to bake, this stuff is the best. The vanilla you buy in the grocery stores at home doesn't even compare.