If you start at like 9am, you can probably finish Koog-Zaandijk by 1-2pm, so after that you can tour Red Light District and Chinatown and visit some coffee shops.
Head off to Koog-Zaandijk, which is a 6 euro roundtrip, 1 hour from Amsterdam Central. This is the city of windmills, and there are a lot of mills to see and they are pretty interesting. There is a spice mill, cheese mill, clog mill and they will demonstrate how the goods are made and I found the cheese one to be rather enlightening. It is also a beautiful place.
