In Apeldoorn you can find Palace ‘t Loo. Have you visited since the renovation?
You can learn a lot about the Dutch royal family and there are lovely gardens.
Lovely Leiden.
A short train ride from Amsterdam you can find the city of Leiden. It’s the birth place of the most famous Dutch painter, Rembrandt van Rijn. It used to be the home of the Pilgrim Fathers, before they went to the USA. There is beauty around every corner. Well worth a visit!
Naarden, a fortified city is a hidden gem just a short drive from Amsterdam. The fortifications date back to the end of the 17th C and are well preserved. You can visit among other things the Vesting museum and learn about the military importance of Naarden.
Or you can take a stroll through the pretty center.
The Gele Loods(Tourist Information) has a nice display about the role Naarden played in the Hollandse Waterlinie and they have a very informative movie too.