my hometown
you definitely need to go to the "Reeperbahn" - the world famous redlight district. tons of bars and clubs there.
but avoid the hookers and stripclubs - it's all scam.
if you can go out saturday night and then go at 6 am down to the "Fischmarkt" - good place for breakfast and party some more.
and of course you should do a harbour cruise - it's impressive