Actually, though I'm glad to have brought up this undoubtedly useful topic, as far as my situation is concerned, I might have found a solution. I might have occasion to go to Cologne (work-related, it only dawned on me today that it might be well worth doing), so the easiest thing would be for me to return the car at Gare du Nord and hop on a direct train to Cologne, spend a day there and then take another train to Amsterdam. The road trip aspect is likely going to suffer a bit - or I might just grab a car upon arrival and head down to Brussels for a day (kinda want to go to Europa Park) and then back up to Amsterdam.