A possible alternative to Rotterdam is Den Haag, but it depends on which has more attractions. For example, there is no substitute for the Mauritshuis and other museums, but Rotterdam has better architecture and perhaps modern art. There is a public (road) bus to Kinderdijk also, but this is a lengthy outing, almost half the day.
This is kind of a short vacation, but your schedule looks fine. Getting to BRU early for a flight can be annoying and expensive, so read up on the train to the airport. That would be the problem with a third night in Antwerp. It would be a shame to stay at the airport hotel! But many people forget about that when they buy the cheapest plane tickets they can find. The Ancient/Modern art museum in Brussels is superb, and the Magritte museum unique, if he interests you. I would choose Bozar (museum) if you don't care about Magritte, or the Coudenberg palace-cellars museum, too. Many beautiful churches in both Antwerp and Brussels.
You are right to give two nights to Antwerp, a wonderful destination. Alas, the big art museum, KMSKA is closed for years of renovation, but there are at least eight other world-class museums in Antwerp. Half kidding, if you get bitterballen in the Netherlands, you would go to Desire de Lille in Antwerp for Smoutebollen to compare. There are so many restaurants in Antwerp that there's sure to be someplace open for Christmas. But many of the finer places will be closed, so I suggest you do some research now if you're going to make a reservation.
Although the riverside tiny castle, Het Steen is now closed, you can walk around it to find a touching plaque thanking the Canadian (?) regiment that liberated the port of Antwerp and city. There's a similar plaque in the Brussels Cathedral. You'll find a touching war monument in the main post office and train station of almost any city you go to. Near the KMSKA in Antwerp, there's a street plaque about the bombardment of Antwerp.
