I was there a few years back. It's absolutely nothing like Amsterdam, and for the most part is pretty boring.
A bridge in Rotterdam.*
A canal in Rotterdam.
There are quite a few
coffeeshops, a good shopping bit, the New Year fireworks at the river are awesome, and bits of it are pretty, but that's about it.
Have no idea why I stayed there tbh - ended up getting the train to Dam every day - was money that could've been spent better! Did end up making friends with a couple of guys from Luxembourg who were in the hostel though, so it wasn't a total loss. Got a bastard of a cough and spent a chunk of New Year's Day in my bed rattling like a bag of bottles, but that was about all that put a downer on things.
Coffeeshop-wise, I was in:
Sky High - Was the first place I visited. Was crowded with young locals - I decided since I was in already I'd just go for it. Was choking for a smoke too after the travel (from Glasgow Prestwick-Brussels (Charleoi)-Brussels-Rotterdam) so bought whatever it was, sat down and rolled away. Could feel the eyes on me, but just puffed away nonchalently, even though I was cacking myself

, then headed back to the hostel at Olympic walking qualifying pace and did my smoking there for the night.
Nemo - This kinda became my 'home' CS during the trip. Would stop in there every morning for a gram or two and a few coffees. Very fresh and clean inside, is Moroccan-run, and has a bouncer on the door most of the time. The bar and smoking area are separate, and the interior is decorated in such a way that you feel like you're sitting inside a scene from Finding Nemo.
Think the weed was pre-weighed, but I didn't get a bad bit all week. They also have a pest of a parrot which can imitate the squeak of the smoking room door opening.
Pluto Not much seating in here iirc - just bought and flew a couple of times. Pretty much next door to Sky High. Can't really remember much else.
Since I was commuting most of the time, I just stuck to the shops closest to the hostel, so that's about all I know. There are a good few around though.
In short, I'd probably just stay in the Dam, unless you really like boats.

There's really nothing there smoke-wise that justifies the E20 or so that a return train ticket would cost, plus a good couple of hours travelling time in total. It was freezin' too, btw.
*The bridge is awesome btw.