The bright orange skywalk runs for some 600 metres (1,969 feet), with a heart-stopping "airbridge" section linking the city's World Trade Centre to a department store, at 29.5 metres above street level.
The walk, which is open until June 24, also features wind turbines, solar panels, art galleries and a drone landing pad.
If the city authorities give approval for a full-scale rooftop village in future, it is expected to include crops and tiny houses built of sustainable materials.
With around a third of its land lying below a sea level that creeps higher each year, the Netherlands has become a world leader in adapting to climate change.
The urgency is even greater for the Dutch given that the nation's 17 million inhabitants are squeezed into Europe's most densely populated country after tiny Monaco, the Vatican City, Malta and San Marino.
Known for its architectural daring in the decades after it was flattened during World War II, Rotterdam itself is something of a pioneer for the Netherlands, which only uses some 1.8 percent of its roof space.
Transforming the city could take decades but van Geest says he is "convinced that this will become a reality".
As the Dutch population becomes increasingly urban, "space is becoming a rare commodity in the city, so we will have to exploit the roof", he added.
Rotterdammers are enjoying the change of perspective.
"It is a unique opportunity to see Rotterdam from a higher distance," approved 69-year-old resident Harry Schouten.
The "Rooftop Days" have been going on for six years and the latest highlights some of the most successful ideas for a climate-adapted future.
These include the "Rooftop Field", a 1,000-square-metre area on the sixth floor of a building which grows vegetables, fruit and edible flowers.
Founder Emile van Rinsum, director of the Rotterdam Environment Centre, said his organisation created the field nearly 10 years ago on the roof of the building where their offices are located.
"It's really nice" to work a few staircases away from such a green space in the heart of the Netherlands' second city, he said.
One of its main purposes is for storing water, as climate change makes seasonal rainfall levels increasingly unpredictable.
"On this roof, we can already store 60,000 litres of water," Van Rinsum said.
Part of the produce grown there is delivered to eateries in Rotterdam, while a restaurant set up near the field is proving "very popular".
"We call them 'intensive rooftops' on which you can walk or, for example, grow food as we do, and that is very important for a city," he said.