Next month there is the rooftop festival in Rotterdam (Rotterdamse Dakendagen) and part of that is the Rotterdam Rooftop Walk. If you want to join please buy your own ticket at

https://rooftopwalk.nl/

(i bought a ticket for the 13:00-13:15 time slot on saturday 18th). The walk takes max 1 hr. After that we can further explore Rotterdam. Transport will this time be by train. Questions/comments=> on the social calendar page of Eindhoven (in BASH) are all the WhatsApp group links (use weekend event for this one).