If you have half a day to spare in the Netherlands, get out of Amsterdam and come here! You just need to hop on the train to Rotterdam (make sure you buy the supplement for your train ticket), take the tram to Erasmusburg to board the ferry that takes you to the entrance of the kinderdijk windmills.
You don't need to buy tickets if you don't plan to visit the museum which is essentially the inside of two of the windmills. What I suggest is renting a bike to go around all the windmills, relax, take photos and have fun. Biking in NL is very pleasant as the country is as flat as a pancake :-)