Good choice - it is a fantastic country (not Dutch, but had family living there for a while). Efteling and Madurodam already mentioned are great choices. In addition, tulip fields may not be an option, but you might enjoy the Aalsmeer flower auction. This is NOT a tourist attraction, it's a major business, but it is fascinating to see just how industrialised flower growing, sales and export is in the Netherlands. Tours run daily, and seeing the Dutch Auction process is great.
Amsterdam generally is a wonderful city - Pancakes at The Pancake Bakery on Prinsengracht, a boat tour on the canals, lots of activities in Vondelpark, the NEMO science museum and the art museums are all fantastic.
Away from Amsterdam, I would recommend the Kröller-Müller Museum in Hoge Veluwe National Park. Even if you don't go onto the museum, the park is great, and there are free to use bikes to cycle around.
Also fantastic is the Beekse Bergen safari park and zoo. It's wonderful, and I believe you can now stay overnight in cabins on site.