Many years ago, all I can remember was the very old centre was pleasant to wander around (along the lines of the Shambles in York) and a park north of the station when it got too hot in the city (which won't be a problem in November!). You'll probably be too early for the Christmas market but the shops in the old part would be worth looking in for Christmassy stuff.

someone else asked about Bremen a few months back, Bremen Transport is here and you need a number 6 tram from the airport to the centre.