Basel where we board a river cruise ship for 2 weeks on the Moselle and Rhine, ending in Amsterdam.
We are doing the reverse of your cruise - leaving from Amsterdam and ending in Basel. We're going with a group of 40 couples from our country club, so it should be a blast. From there, my wife and I are taking a train to the Champagne region for 2 days and then to Paris for 3 days. Really looking forward to this as I have never been to Europe.
If we don't spend all of our money in Champagne, we get back for our daughter's wedding on October.