Depends on what you fancy. Hamburg is a bit "gritty", I guess a typical port town. Boasts the famous Raiperbahn district (not one to bring the wife to on a romantic weekend) and a lively pub scene in inner city.
If you are going to have a long night in Hamburg, make sure to finish it off in the Fischmarkt, the fish market which opens approx 5 am. Many revellers will finish off their night with a herring washed down with some local Korn schnapps.
In my opinion, Hamburg is more a destination for a weekend away with the lads, rather than a family or romantic destination.
Ubquitous is right about Lubeck from what I hear, apparently famed for its Christmas market, which probably runs from November.