I am ending a cruise in Rotterdam and would like to go into Amsterdam for a few days. What is the easiest way to get from the Rotterdam cruise ports to downtown Amsterdam? Is there a train? Bus? or is taxi the best way?
Take a taxi from the passenger cruise terminal to Rotterdam Centraal (train station) - it is a 10min drive only -, and take a Intercity Direct train from there to Amsterdam Centraal - trains depart every half-hour and the trip lasts 44 minutes.
You can lower your costs even further by walking to the subway station near the cruise terminal (400m or so), taking a subway to Rotterdam Centraal and then a train, but though the subway is modern and have lifts, it is maybe not that comfortable if you have large piece of luggage.
Taking a taxi from the cruise port to Amsterdam is possible but it will be very costly - € 300 or more for a on-spot cab, unless you scheduled some car transfer. It is not needed, in my opinion.
You're on the right track. You don't really need train schedules in advance, since they run so frequently, but can see How to Look Up Train Schedules Online. Buy tickets in the station.