Being a Dutchman let me recommend The Hague (Den Haag) and Haarlem over Amsterdam. Unless you wish to visit the Rijks and/or Van Gogh museums.
Other Dutch cities that will deliver are: Den Bosch and Utrecht.
Maastricht, as mentioned, is a worthwhile contestant too, but depending on your mode of transport, perhaps a little off the route.
Across the border in Belgium, Brussels is always a good choice, but so is Bruges (Brugge).
Wover