Been to Groningen and liked it. I was a freelance IT teacher at the time and had to do a couple of days up their at a company. It was quite funny. I couldn't understand their dialect, so I had to agree that they spoke in normal Dutch to me. The two days were conducted totally in Dutch and the English language was not allowed. It was tough for me, but a great experience. Such nice people.
Leiden is a nice place. I am just trying to recollect. I saw Queen twice live in Holland. One was at Rotterdam Ahoy and the other was in Leiden, but can't remember the venue (the place rocked that night).
I particularly like Gouda.