The best time to visit Rotterdam is late spring to early autumn (April to September), when the weather is mildest, the days are long, and the river terraces, festivals and rooftop bars come alive. Summer (June to August) is the liveliest and busiest, with big events like the Summer Carnival and North Sea Jazz, while spring brings mild days and a nearby tulip season. Winter is cold, grey and short on daylight but quieter and cheaper, and the modern indoor sights — the Markthal, Maritime Museum and Kunsthal — are good rainy-day options. Rotterdam's weather is changeable year-round, so pack a rain jacket whatever the season.
Rotterdam is very safe, but as in any big city petty theft happens in crowds — at Rotterdam Centraal, in the Markthal and on packed trams and metros. Keep bags zipped and in front, secure your phone, and stay alert if someone jostles you or creates a distraction.
Bikes have their own marked lanes and move fast, and visitors often mistake a red cycle lane for the pavement. Look both ways before crossing a bike lane, never stand or stop in one, and be predictable when walking near cyclists.
Rotterdam sits on the wide Nieuwe Maas with open quaysides and harbours, and many stretches have no railings. Take care walking or cycling near the water's edge, especially after dark or after drinking, and keep children well back.