For a very beautiful view over the Elbe and Hamburg's harbour is worth the "rise" to a kind of observation deck above the train/subway-station "Landungsbrücken".
Leave the station at the West exit and do not just use the stairs down. Go right site to the top and you will reach the viewpoint close to the youth hostel.
Especially beautiful it is the evening when the sun goes down.
Those who want to make it the typical "hamburgian way" can buy before downstairs at the harbour some herring rolls and a bottle of Astra beer. Then buy and enjoy this perfect view. :)