first of all im guessing that ur landing at 6pm not 6am(coffeeshops dont open before 10am usually)

that said ur looking at roughly 20mins each way on the train (taxi might be quicker, i dunno i always take the train) once youre in amsterdam you can find a coffeeshop right at the top of the damrak (main drag leading from train station) so it would all depend on how quickly you can get off the plane, wether you need to collect your luggage or not, and how quickly you could get back through customs on your return to the airport. personally i dont think its possible, and to do it at all you'd need [U]everything[U] to go your way.

still, if you do pull it off it would be a great little buzz :rasta: good luck if you decide to try it

oh, and tourists can still smoke in the coffeeshops, that change in the law, if it even happens, wont happen till the beginning of 2013(edit)