Rotterdam is 65 minutes from Amsterdam Centraal and this will soon be down to 35 minutes with the new high speed line opening soon. We went for the day and went to the Euromast Tower and a harbour tour.
Despite lots of building work outside the station (just like Amsterdam) the tram station is nearby and its a 10 minute ride to the Euromast Tower. Its views cover the entire city and there is a restaurant there that unfortunately doesn't revolve but still has great views. The Harbour tours by Spido company depart from the large and impressive Erasmus Bridge.
Your Amsterdam strippenkart ticket is valid on Rotterdam trams.
We did Rotterdam as part of our day tour. Unfortunately we did not see that much of Rotterdam except for all the bridges, but lunch was good