Yes there is plenty to see in the Netherlands (and I am Dutch) but I think it would be very nice to combine it with a city in Belgium. However, why in the world would you want to include Brussels if you have such a tight schedule?? Read this thread about Brussels, Bruges and Ghent.
Bruges, Ghent, Antwerp are all very nice. Antwerp is easiest to reach from Amsterdam. For Ghent or Bruges you need to change trains in Antwerp.
Also, Luxemburg city... why? Sure it's a nice city but not more worthwhile than Antwerp (Belgium) or Maastricht (Netherlands) for example, both of which are easier to reach. If you want to go to Luxemburg (country), then choose a lovely town like Vianden instead, with a nice castle and very steep rising streets. I don't know how easy it is to reach this town by public transport though.
For tulips the suggestions above are good. For windmills consider Zaanse Schans, in Zaandam near Amsterdam. Bus tours from Amsterdam go there, or rent a bike and go there yourself.
For my recommendations on Amsterdam go to my website > Travel > Amsterdam Tips.
Veel plezier,
Cécile
Amsterdam
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