This weekend we enjoyed some quite time at the Hoge Veluwe National Park. I know, a bit cold, but it was a special occasion...you can get a free bike there and see more of the park (the bike paths get right through the middle of the forest). We got some exercise for sure!
And in the middel of the park you will find the Kröller-Müller museum. They have an impressive collection of Picasso, Mondriaan, Seurat, etc. They lend many pieces so you can check on their website before going if they will be at the museum. I was totally impressed by the Van Gogh collection.
They also have an enormous collection of sculptures in the garden that you can visit for free (there is a fee for the museum, several discounts are available or you can go in for free with the museumkaart).
The park is located near the city of Arnhem. You can get there by trein and bus. On the park´s site you will find a lot of useful information.
Hope this inspired you to do some sightseeing!