stay in coyoacan if you can. a B&B might cost you 60 a night, but a crappy hostel by the zocalo is $50, and coyoacan is nice, safe, and full of good restaurants. in coyoacan, go to frida's house (casa azul), and the diego rivera museum (which I haven't seen) the zocalo for sure. take a trip to the pyramids. it's usually combined with some other cool stuff. ride the metro.