Spending 5 days in Amsterdam, should we do The Hague on one?
The question might answer itself if you make a list of what you would be most interested to do and see in Den Haag. When I visited for a day, I had very specific reasons for wanting to go (to see the Panorama Mesdeg and the Mondrian collection in the modern art museum). While I was there, I stopped by to see other sights of artistic and historic interest. Many people go just to see The Girl With The Pearl Earring in the lovely & small Mauritshais Museum, but the painting often is on loan, so if that is high your list, make sure it there. Other people I know have gone to visit the International Court of Justice, but I have never been clear about tours or when it is open. I would imagine you can only go as part of a guided tour.
If your main interest in travel is simply strolling in historic centers. the one in Den Haag is fairly small, and most people would probably go to Haarlem or Delft instead.