Oh man. Depends on what you're interested in. Amazing museums all over the place. The farming province of Friesland is gorgeous, and every village has an historic church. The village of Saint Annaparochie for example has the old church and the new church. The new church is 15th century. It's hexagonal and beautiful.
Other than that you should check out companies that do canal tours. You can do a "bike and barge" thing across the country where you can travel the internal canals and get off wherever and bike about, stay in a little place, and get on the boat again later that day or the next day.
Van Gogh museum is a must, though it is somewhat diminished right at the moment as the Chicago Art Institute has a Van Gogh exhibit on right now and they have at least a hundred pieces from that museum.
Afsluitdijk is also worth going to. Huge dike built in the early 20th century that helped the country claim hundreds of hectares from the ocean and improved travel immensely.
There are also plenty of very old castles to see, you can google them. Especially Valkenburg and De Haar.
Go to Lake Ijssel, which is now a lake that was formed after closing off the sea. It has some of the most iconic buildings in the Netherlands. It's absolutely gorgeous, and you'll find the history and engineering of it fascinating.
Rijksmuseum, obv.
Also, the Cathedral Square in Utrecht. Absolutely stunning.
Go to the town of Delft and see all the blue and white pottery. It's a charming place.
If you're in the area, Binnenhof, The Hague, is very interesting. Historic interest.
Rotterdam is also interesting for a number of reasons. It's the largest port in Europe, and a port town always has interesting things as well as amazingly delicious sea food. It was also bombed like crazy during the Second World War, so it's one of the most modernly designed Dutch cities as they started with a nearly clean slate.
And there's the obvious attraction of Kinderdijk. That's where you'll see all the Dutch windmills you ever wanted to see. They move a lot of water up a long way. The Dutch manipulation and control of water is amazing, even going back hundreds of years.
Depending on when you're there, find a tulip farm to tour. They almost all do it, and you'll find the scope of tulip growing stunning.
You'd be surprised at the national parks, especially Nationaal Park De Hoge Veluwe. You can do trail riding on horses, and it's gorgeous all year around.
Okay, sorry. I love this country of my heritage, I could go on a long time. The really nice thing about it is that it is such a small nation (by North American standards) that you can see so much of it in a small time. Plan to get on bicycles at some point.