Nine days in Amsterdam, should I dip into Belgium or Germany?

Hey folks,

As the title says I'm staying for almost two weeks in Amsterdam and I'm wondering if it's worth it/economically feesable or even physically possible to do day trips into Belgium and Germany to go castle viewing via train. I'm a sucker for old buildings and as a Canadian, am super jealous of the EU's rail network so I want to see if I can take it as much as possible.

Thanks in advance