Hop on a bus in Rotterdam and you'll be transported to the past. Around 1740 about 19 sturdy windmills were built in Kinderdijk outside of Rotterdam. They have been well preserved to the present day and are even listed as a Unesco World Heritage Site.
I suggest you go and visit on the weekday as there will be less of a crowd, or on a rainy day like I did! Regardless of the weather, you can't help but be in awe of these old windmills and their power.
A nice walking and biking path winds around each of the 19 windmills so you'll get your exercise at the same time.
More Information: http://www.kinderdijk.com/index.html