The Grote Kerk, the enormous church in the middle of Haarlem, is very interesting. I was underwhelmed by the Franz Hals Museum but many people like it more. Walking around central Haarlem is fun and much less crowded than Amsterdam.
The Hermitage Amsterdam has an exhibit of 18th century Italian art if that interests you, on loan from the "real" Hermitage in St. Petersburg. I also got a lot out of the permanent exhibit, "Portrait Gallery of the Golden Age," which gave a lot of understanding of how Amsterdam set a different template for governance while most of Europe was still all about king vs. king. The Dutch got a head start on the idea of "civil society" before the rest of us.