I am fairly sure you can go to the tour desk on the ship and cancel them, they don't charge you until you pick up the tickets anyway, so till then I'm sure you can work something out...

I wouldn't waste my money on prearranged tours in Amsterdam or Bruges, these cities can be toured with a lonely planet guide and a train ticket. Especially Amsterdam is really easy to tour, it is just an hour from Rotterdam by train.

How long do you have in Rotterdam and Zeebrugge? The port that the ship docks in i pretty much in the city center and probably only a EUR10 cab ride from the shore. The city of Rotterdam has their own website for the cruise terminal:

http://www.cruise.rotterdam.nl/terminal.htm