If you fancy an - ahem - authentic experience go to the Jolly Rodger which is a St Pauli fans club.
We went to Altes Madchen as well which was class for a tasty meal. Great place for a stag.
Hamburg stadium is a bit out of town and they have an undeniable Hun love in going on, much like St Pauli have this left wing Irish love in on the go but St Pauli's stadium is much better located for the peeve before and after.
I stayed in the pyjama hotel/hostel on the reeperbahn, if st pauli are playing away from home the jolly roger or the zoo pubs are best to go to, the reeperbahn is a great place to be for a weekend, but watch what strip bars you go into as the bouncers can be a bit heavy handed, play it safe an go to the doll house, its€12 euro to get in and€5 a drink but being honest you don't stay to long in these places anyway, a tip for you when landing take train from airport it only costs €11 for upto 5 people an takes 20/30mins. A taxi will cost you about €40/50 and take same time, the pass lasts for 1 day and can be used for other trains about the city
I had my stag so there, it's a good place to go. It's worth going out round the Reaperbahn one night but there's other good areas as well and one night is enough. Can't remember where they are but should be easy enough to google.
My mates hired a wee boat with a bar that gave us a tour of the harbour and some special onboard entertainment. Well good laugh and would recommend it. Kevin Keegan played for Hamburg so I also came back with a Keegan wig and football top.
I was broken at the end and felt like I had been to war by the time I made it home.