when you come to berlin, there's a few spots worth checking out: görlitzer park is a nice hangout in the evenings and on weekends, mauerpark has a flee market worth checking out. in general you should see kreuzberg (kottbusser tor) and neukölln, the hipper parts of town. clubbing is always an option in both, but friedrichshain is good for that too. if you're up for some traditional head nodding, you go to a party in cassiopeia or bohannon. if you want to exerience some real craziness, you go to sisyphos, where people get in on a friday night and go home sunday morning. don't expect to find any cheap records here, berlin is a digger's hell. you can also drop me line when you're here, i'll show you around if you like.
i wonder why you would want to go to frankfurt. it really is a waste of time if you don't have personal business there. a few medium sized sky scrapers and that's it. i'd rather check out leipzig for a friday night. great city with a beautiful historic center and communist flair. or you could think about a trip to poland and go to krakow for a day or two. worth the effort.