Delftse Poort is a large office complex right next to Rotterdam central station. The ground floor public spaces are open to anyone, and you can find some really nice spots to sit and work from. Some tables are marked as "Tenants only", which cannot be used. However, if you see a sign saying "Flexible working" then you can use the space. There is also a small cafe if you head up using the escalators. Restrooms are located upstairs as well. Fast WiFi, comfortable chairs, and power outlets.