We passed through Rotterdam on our way from Paris to Delft. Rotterdam was bombed out by Hitler in WWII, so has been rebuilt almost from scratch. The result is some ultra modern architecture, some of it quite whimsical.
We had a few hours in The Hague on our way from Delft to Amsterdam. We had time to wander about and visit the Mauritshius, an art museum with an amazing collection of paintings by masters of the Dutch Golden Age, including Vermeer and Rembrandt.