My husband and I have just returned from a four night stay in Hamburg and what a lovely city. If visiting do go on the sightseeing buses, take in all of them if you have the time, a great way to see the city. If taking one of the boat trips on the lake or around the canals etc., make sure you ask if they provide headsets with the commentary in English.
We purchased the Hamburg card on arrival at the airport which covered our travel into the city, all buses, trains and even the local ferries on the river.
Another must is a visit to Miniature Wunderland, but make sure you reserve your tickets in advance and go for an early slot. We arrived at 9.20a.m. and left at 5.30p.m.! It is an amazing place with so much to see.