The Hague (Den Haag,) has plenty of lovely things to see. Some nice museums I would recommend are: Mauritshaus (an art museum hosting the famous Girl with a Pearl Earring), Louwmanmuseum (if you happen to also love cars,) and Escher in de paleis.
Particularly in note, since we’re on the UK rail forums is the Haags Openbaar vervoer museum (The Hague public transit museum.) it’s only open on Sunday afternoons, but has an extensive tram collection and also offers a ride in a historic tram through the city. The volunteers that work there are all knowledgeable about what’s what and are happy to open up trams being exhibited to let you see inside. Also feel free to ask even the most technical of questions about any of them.
I visited Eindhoven during Dutch design week, where the biggest highlight was the graduate show of its design academy. The city itself isn’t particularly my most favorite to be honest. It’s not bad, just modern and somewhat boring. It does have a few nice museums, but the Philips museum was a letdown.
I assume that you’ve visited Amsterdam and Rotterdam as you mentioned having visited the big 3, the last one of which I can’t guess. But some other nice cities to visit, to give you some more options are: Leiden, Delft, ‘s-Hertogenbosch/Den Bosch, Utrecht (particularly for the national railway museum, take a train to the museum itself!), and Haarlem