I would first have a good breakfast at Baker & Moore. Vegetarians and vegans can indulge themselves there with tasty breakfasts and good coffee. When you have recovered a bit, you should go for a walk through the city. If you walk, the city will come to you. I can recommend the Rotterdam Routes app. There are many walks to choose from, so you can also get off the beaten track. We recently did a street art route with the office, and the app gives you information about what you see.
Also lovely: walk over the Luchtsingel to the rooftop park. That pedestrian bridge was one of the very first crowdfunding projects in Rotterdam. The names on the boards of the construction belong to the investors. The municipality is now conducting a survey via a QR code on the bridge, where you can give your suggestions on how to change this place. More green? More seating? It’s typical of Rotterdam to first research what people want and then experiment to see what works well. You don’t always know that in advance when you start something new, and that’s the beauty of it.