Late spring to early autumn (May to September) is the best time, with the longest days, greenest parks and the warmest, if still changeable, weather for harbour trips and lake walks. Hamburg is famously rainy and windy year-round, so pack a waterproof in any season; winters are cold, grey and dark but bring cosy Christmas markets.
Hamburg's maritime weather is changeable and often wet and windy, even in summer, so carry a waterproof layer and expect sudden showers. The wind off the Elbe and the North Sea can be sharp at the harbour and on the ferries, so dress warmly for open-air sightseeing.
The St Pauli and Reeperbahn nightlife area is lively and mostly safe early on but gets rowdy, crowded and seedier late at night, with heavy drinking and pushy touts. Keep valuables secure, watch your drinks, avoid clip joints and the fenced Herbertstrasse, and travel back in a group or by official taxi.
Hamburg is a keen cycling city with busy bike lanes that often run along the pavement and are easy to stray into on foot. Look both ways for fast-moving bikes before stepping off a kerb, and keep clear of the marked cycle track when walking or waiting to cross.