A visit to the windmill complex at Kinderdijk, a UNESCO World Heritage site since 1997, is a must when you head to the southern part of Netherlands.
Kinderdijk is pretty far from Amsterdam, so my first suggestion would be to get up early if you plan to go here and make the most out of your trip! From Amsterdam Centraal station, you should go down two stops later at Utrecht Centraal. It takes around 30 minutes only. Tickets can be bought at the station, or on their website where you can check the price and schedule ahead of time.