Speicherstadt, Reeperbahn, Landungsbrücken, the old tunnel crossing the Elbe, the shopping possibilities that are south of the central railway station and go all the way down to the bottom of smaller Alster.

If you're into airplanes, you could make a tour of the Airbus factory in Finkenwerder. Unfortunately, their ferry boats no longer stop there, making getting there a lot harder than it used to be. I never did the tour due to the lack of time.

There are far more things to do in Hamburg, but you'll probably need more time. If you're traveling with kids, you might go to Miniatur Wunderland. It's a huge display with miniature cars, trains, aircrafts and worlds (apparently it's over 1,500 square-meters large). For the visit, you'll need however a lot of time.