christ, you might want to get some comments from the Search box for this forum. The key to understanding the reason to go near Brussels is the number of close-by cities that are easy to get to by unreserved, frequent local trains, at low cost and without advance purchase. It is, in fact, unnecessary to change hotels - unless you have some high-priority reason, like seeing Bruges before the daytrippers arrive. (I'm not that entranced by Bruges, but I'm in the minority.) Don't forget to research Monday closures.
Please explain how you picked Rotterdam. There are alternatives to Kinderdijk. Rotterdam is a swell place, but there are so many attractive cities in the Netherlands that you didn't mention. And many of them can be visited by easy daytrips from Amsterdam. These include Leiden, Delft, Den Haag, Amersfoort, Haarlem, Alkmaar, Utrecht. (I have no information on New Year's Eve.)
In Belgium, you need to look into Brugge, Gent, Antwerp, and Leuven, with lower priority given to Lier, Mechelen, and maybe Turnhout. Of course, you can't visit all of those in just a few days, and you don't need to. But you can pick the places that interest you most. I find Antwerp much more attractive than Brussels, and would sleep there-not Brussels. However, while the biggest art museum (not the only one!) in Antwerp is currently being renovated, it may be easier to see a lot of major art in Brussels. But for daily living amid fragments of history, Antwerp is pleasanter. (Opinion.) And there are plenty of other nice museums in Antwerp. Some of the big KMSKA museum's art is up at the MAS museum, for example.
I don't quite understand why everyone pairs Brugge and Amsterdam. Brugge is not that far from Brussels, but does require a train change and a substantial local ride from either Antwerp or Brussels. I would be satisfied with a trip just to Belgium, where the food is better than in the Netherlands. (Amsterdam may be an exception to that statement.) But you're right to buy an open-jaw air ticket. Although it doesn't have canals, Antwerp is equally good for a young man's first (?) trip to Europe-in terms of English usage and different street culture. Antwerp even shares some of Amsterdam's ills.