6 hours is just enough time if you have to make it back for an international flight. you need to be there at least an hour (they say 1.5 or 2 but thats not really necessary unless its peak season) before the flight. I went to the city on a 5 hr layover and it was barely enough time. Like someone already said, its 20 mins each way from schippul to centraal. Boat tours can take a half hour or more, but there are some that you can use as a public transit and only pay once. For example there is a boat that goes from the central station to the ann frank house, you can get off and tour the house, and then take a different boat back to the station. doing anything more than that or something like it would be cutting it close. if you are into the whole coffee shop thing, you should go while you can because they are making it illegal for foreigners.
