As a previous poster mentioned Blankenberg is a nice place and the tram stop is a short walk from the port gate, turn right out of the port and you will find the stop near a submarine moored nearby.
Rotterdam to the town is walkable, turn left out of the port and go to the road junction, turn left again and cross the road bridge, follow the road on the right hand side which takes you past an open air museum with old railway trucks on a quayside. Continue walking and you will come to a shopping centre. To return just retrace the route, worth looking at Google maps of the route beforehand. There are also trip boats across the river from the the ship moors up.