any good tours of the city we can do after the tennis?
If the weather is good, a Hamburg "must" is a harbor boat trip. They start at the Landungsbrücken (take the train to the stop with the same name, e.g. train S 1 or U 3).
There are double decker busses that do city tours (like in most big towns). You buy a ticket for the day and then you can step in and out of the bus at the various stops, like the Rathaus (town house, nice tours inside) or the Michel (biggest church in Hamburg). A good place to start for that are the Landungsbrücken as well.
Another fun thing to do is to get a "look from the top" by going into the air in a hot air balloon. The balloon is chained to the ground, so you go up to about 150 m and can enjoy the view over Hamburg (only in good weather!). Costs 15 €. Train stop Steinstraße (train U 1).
Walking around the Alster lake in good weather is very popular in Hamburg. The lake is about 5 min. away from the tennis stadium, so if you need a break it's a good place to relax.
In the evening, pretty much everybody goes to the Reeperbahn (train S 1 to the station with the same name) for dancing and drinking. Also nice for that is the Schanzenviertel (train stop "Sternschanze"). For those who are in Hamburg during the Qualis: the Harbor Festival is going on, lots of drinking and partying, starting (again) at Landungsbrücken.
Should be enough for a quick overview.
I'll be around for the whole duration of the tourney, together with some international guests. Anybody who feels lonely, feel free to join us.