Stuff to see..check out the cube houses near Rotterdam Blaak Metro station (theres also a big building which looks like a pencil).The cube houses, have on that you are able to have a look in. Pretty weird but cool! Theres also often a market in this area which is fun to check out.
The Euromast is a little bit out of the city (it is walkable), but cool to see (A decade ago you were able to abseil down it which is great fun, but not sure if they do it anymore)
The interesting aspect of Rotterdam compared to the other dutch cities, is that during war time it was bombed heavily, so out of all the cities it is the newest and has the greatest variety of interesting newer architecture.
To see some of the older architecture, old style of bridges, windmills, check out Delfshaven,
I generally only go there to shop, but its quite nice to wander round and check out the architecture. almost 6 years ago