Maastricht has a fascinating history going back 2000 years to Roman times:

Before you read the info below, here's what I found about Castles near Maastricht. Castle Witten is outside a town called Wijlre, then a taxi ride to the castle. Castle Erenstein is near Kerkrade, again a taxi ride away from the town. Both towns can be reached by train, according to a book I found called Backroads of Holland by Helen Colijn.

More Maastricht history from the Backroads book by Helen Colijn. Remember this info is from 1992:

Maastricht goes back 2000 years; the Romans founded it as Mosae Trajectum, ford of the Muese. In the Bonnefantenmuseum of art and antiquities you can see Roman archaeological finds. Colijn wrote that the musuem will move within a few years to the Ceramique Park development, whatever that means.

After the Romans left, Maastricht became the seat for Christian bishops. The first one, Servatius, died around 384 AD. There is a beautiful church, St. Servatius, which contains some of his remains.

Several of the Middle Ages churches and walls still exist in the city of Maastricht. A few blocks away from St. Servatius is the basilica of Our Beloved Lady, Onze Lieve Vrouwe Basiliek.

In the 19th century Maastricht became the capital of the new Limburg province.

On a hill near Margraten, east of Maastricht, is a cemetery granted to the US in memory of the 8,000 soldiers buried there who died nearby in WWII.

Near Heerlen is the Thermaemuseum, built on the old Roman Baths.

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> would you recommend a little day "side >trip" while we are in Maastricht to perhaps Belgium, Luxembourg, etc?

Luxembourg is a bit far away for a day trip, about a 6 hour train ride from Maastricht through the beautiful Ardenne mountains. To visit Luxembourg, I think you'd have to arrive one morning and return the next night, and that would be barely enough time to see it.

Closer is Liege, Belgium and Cologne, Germany. I think Koln (Cologne) has an important cathedral, as well as other things to see. I've only been through it on a train.

Liege is another town where I just stopped at the train station for a bit. As you approach there is a big dome on a hill. Also the old section of Liege is pretty neat.

Have you considered doing an overnight train trip somewhere? If there's only 3 on you, you could reserve a sleeper with 3 beds and have some privacy from others. Just a suggestion... maybe go from Amsterdam to Lux ( I don't know offhand if there is an overnight train for this), stay one night in Lux, then return on another night train?

Also Lake Lucerne is Switzerland could be reached this way, though that may be too far to travel.