The big Hohenzollern railway bridge over the Rhine immediately east of the station has a footpath on the south side from which there's a good view of the constant action.
There are various tramways in the area, Cologne itself has a large network, with a long riverside interurban line down to Bonn, which has its own urban system.
Dusseldorf is also close by, another major rail centre. You can make this a triangle trip with Wuppertal.
Beyond railways, the cathedral, next to the station, is one of the most spectacular in Europe. You can rent bicycles and ride beside the river, go into various cafes along there, watch the incessant commercial barge traffic on the Rhine - it's a pleasant city.