Zaanse Schans (***) - With windmills still in operation, you can also visit museums, factories of cheese and clogs, etc.. Take the train to Koog-Zaandijk (takes approximately 15 minutes), then turn left at the bottom of the platform and walk down the street (about 5 minutes). Turn left and you will see a windmill close to a bridge. Pass this bridge and find the entrance gate on your left.
http://www.zaanseschans.nl
Hoge Veluwe (****) - This is a national park in the Netherlands which has magnificent views and yet you are also given a free bicycle so you can travel through the park. In the middle of the park lies the Kröller-Müller Museum, which is a modern art museum containing an important collection of works by Van Gogh, Seurat, Redon, Braque, Picasso, Juan Gris, Mondriaan and others. NS Railways has daily trips to Arnhem station (journey takes 1 hour), then just take the bus (direction "Schaarrsbergen") to the gates of the park.
http://www.hogeveluwe.nl
De Efteling (*****) - is the largest theme park in the Netherlands and it is also one of the major parks within Europe. Efteling opened in 1951, is located in the city of Kaatsheuvel, in the municipality of Loon Zand it originally was create so that children would have a "fairy tails" theme park. Nowadays Efteling appeals to new and old due to its cultural romantic and nostalgic themes and its variety of entertainment. In fifty years the park has evolved from a Fairy Tale Forest Park to a park along the lines of Disneyland Paris. In 1992 Efteling received the IAAPA Applause Award for the best amusement park in the world.
http://www.efteling.com/
Dolfinarium Harderwijk (****) - This theme park focuses on the marine animals: from dolphins, sea lions, seals, rays, etc.. It is a very interesting park that still has several shows with animals. 5 minutes (on foot) from the centre of Harderwijk. This is indeed a place to go if you are in the south of the Netherlands.
http://www.dolfinarium.nl
Keukenhof (***) - Located near Lisse it is the largest flower garden in the world. It is recommended to visit it from late March and early April. The entrance is a bit expensive though.
http://www.keukenhof.nl