Brugges is Fantastic. Antwerp is Fantastic. Leuven is the biggest secret in Europe!
Amsterdam is great. Rijksmuseum is very, very impressive. Van Gogh museum very good. Take a canal, cruise, actually take the private, small boat one, its not much more. If Keukenhoff is still open for flower festival, it much better than I thought (I thought it would suck) but Tulips may be past bloom, it is a half day commitment though as it is 40-60 minute to get out there via bus. Rent a bike and ride through Vondelpark, and even the Museumsplein or the streets if you are up for it. Ann Frank House book a tix in advance, otherwise 3-plus hours to get in, but its good, so book.
I went with my kids so I can't get into the more adult stuff, but if you are untethered, then certainly explore all the sin options.