For architecture, Rotterdam is great. The tourist office has a brochure with an architecture walking tour, which doubles as a free map (their regular map is not free). Don't miss the Cube Houses and the Cube House Museum.
I loved Utrecht, with its unique bi-level canals and great museums, and general great vibe. I also liked Leiden. While Utrecht and Delft also have universities, Leiden was the only one of the three that had a "college town" look and feel. The science museum in Leiden is very interesting. Did you know that EKG's and kidney dialysis are from the Netherlands? I didn't until I went to this museum.
I was much less taken with Delft than most others are, and found the Hague only of interest for its museums (some good, but none made the impression of the Centraal Museum or the music box museum or the rail museum, all in Utrecht).
You'll notice my focus was on cities; that's why I asked about your interest. For many, my destinations would be far too urban for their taste.