After a three day stay in Amsterdam, we would like to visit the tulip fields of keukenhof on our way to Brussels. Any suggestions on what method of transportation?
You need to provide a bit more information in order to help you better. When will you be traveling? Will you have luggage? If so, how much? Who will be traveling? Family of 6 or just 2 people?
Note that Keukenhof is a huge, carefully planned botanical garden that includes dozens and dozens of tulip beds. Some commercial tulip fields can be seen from the border fences of Keukenhof. Are you saying that you also want to walk into a commercial tulip field, which is also "a thing?" I wouldn't try to do that while traveling with luggage.
The obvious method of transport between A'dam and Brussels is by train. But there is no train station near the Keukenhof gardens. You would want to take a round-trip bus or train/bus for the Keukenhof and then a high speed train from A'dam Centraal. You would want to store your luggage either at your hotel or at the train station. Plan on thegardens taking 2-4 hours to see.