Nice ideas folks, but...
Hamburg was only a one-night stopover, arranged when flybe folded and I could no longer fly to Hannover, so neither suggestion is valid. On the day in question I would have been staying in Leipzig, though the game in question would have been in a neighbouring Land, though still accessible using a Sachsenticket on the trains.
I have been to Wroclaw once before and share Ref Watcher's enthusiasm for it (though again this was to have been a one-night pre-flight stopover.) Gdansk, however, is the favourite of the Polish cities I have visited, but curiously I did enjoy Lodz as well, though I'm not sure why!