Most tourists visit Amsterdam or the Randstad region when they go to the Netherlands. There is nothing wrong with that because both the city and the area have a lot to show.
Personally, I don’t like crowds much myself, so when I still lived in the Netherlands I usually went to other parts of the country.
I have always found the Hoge Veluwe National Park and the Kröller-Müller Museum worth visiting. In every season, although we were longer inside the museum in the winter and spent more time outdoors in the summer.