Rotterdam

Windmill riverboat (aka Kinderdijk, but I prefer your name for it!) is quite a bit out of town, so for a half day you'd need to decide Rotterdam or Windmills. I'd do windmills over harbor cruise.

Rotterdam itself is a city that really pops with some study of good guidebooks. It was bombed flat and rebuild modern, so a little guidance in how to understand the architectural styles and history, what they are meant to mean and why, goes a long way to making Rotterdam more than busy shopping malls and exceptionally good Doner shops. It's a cool dynamic place, a little slice of midtown Manhattan with a Dutch signature.