In 1740 a series of windmills were built to drain a polder about 12km southeast of Rotterdam. Today 19 of the Dutch icons survive at Kinderdijk, a Unesco monument. You can wander the dikes for more than 3km amid the spinning sails and visit inside two of the windmills. Admission also includes a film screened at the former pumping station. Canal boats run trips along the dykes. It's a good bicycle ride from Rotterdam (16km); get a map from the tourist office.
A fantastic day trip is by the Waterbus. The fast ferries leave from Rotterdam every 30 minutes and a connection puts you at Kinderdijk, 1km from the first mill. Afterwards, continue by ferry to charming Dordrecht and then return to Rotterdam by train.