Rotterdam is a vibrant modern city. Plenty to do and see, also inside. And it never rains all day long in The Netherlands.
Have a look at http://en.rotterdam.info/visitors/events/ or for a private guided trip around Rotterdam www.holland-is.com.
I recently started a new blog http://www.netherlandstravelexpert.com/
I will use your question for a new topic within a few days :-) about 5 years ago