We will be in the Netherlands for 5 nights right before Christmas before heading to Belgium. We are going to be primarily in Amsterdam but would like to see Rotterdam. How much time is needed to see Rotterdam? Should we spend the night or go as a day trip from Amsterdam? We mostly want to see the architecture but are open to one or two museums too (I assume I may be museumed out after Amsterdam...).
With only 5 days in the whole country, I think a day trip to Rotterdam is sufficient. The tourist office has a free brochure with a walking tour of the architecture; it doubles as a free city map (the regular city map is for sale there, not free). The walk takes a few hours, and you will want lunch, so those plus a museum will fill a day nicely.
Don't miss going into the Cube House Museum (not covered by the Museumkaart, but cheap).
With and still in the mood for museums you can spend certainly at least a day in Rotterdam. Museum Boijmans van Beuningen and the Kijk-Kubus, the Show Cube of the Cube Houses as already said are worth a visit, but also to my opinion the Maritime Museum.
About architecture and more in particular Modernism the UNESCO listed “Van Nelle Fabriek” is worth to see, but have no idea if there are tours of some kind. Close to Boijmans you can visit Huis Sonneveld and Chabot Museum located between more modernist houses.
Historical Delfsthaven is not big but nice with also the Pilgrim Fathers Church. Schiedam for it´s typical windmills for making Jenever (Dutch gin), one is open to visit.
Rotterdam is further a good base for visiting the windmills of Kinderdijk and the Maeslant Storm Surge Barrier near Hoek van Holland.
As Rotterdam is on the way to Belgium I think it is more efficient to stay there in case the place proofs of more interest than just a superficial visit. You can sleep in the former ocean liner “ss Rotterdam” and climb the Euromast for a view of the city and harbour.
Please do not overlook the Cogels Osy lei in Antwerp, also known as Zurenborg. Right on a tram line, and good for three hours on foot (interiors not public.)
Rotterdam's great harbour ranks in the world's top three for volume of trade. Boat tours give the best viewpoint. Some, in the summer, go all the way out to the ocean where the modern container operations are located; the shorter cruise of the inner harbour is fine on its own. There's also a moderately interesting maritime and harbour museum with a collection of river boats moored nearby.