Be sure to get Rick Steves Amsterdam, Bruges, and Brussels. Despite the title, the book now also covers Ghent and Antwerp, as well as many daytrips in Holland from Amsterdam, complete with all the logistical details. Read about the various options and decide for yourself what interests you the most.
Personally, I loved Antwerp, like Bruges a lot (I was there in April 2002, and it was not that crowded), was slow to warm up to Ghent but in the end liked it, and did not care for Brussels. The undisputed highlight of Brussels for me (really the only highlight) was the Art Nouveau tour with ARAU: http://www.arau.org/en/tours
However, be aware that opinions of these four Belgian cities are all over the place. You can stay in any one of them and see the others as daytrips, so you aren't restricted if you don't like the one(s) you picked.
Three places in Holland that I liked, and that Rick covers sparingly or not at all, were Utrecht, Leiden, and Rotterdam.
Utrecht has some interesting museums. My favorites were the Centraal Museum, with its mix of everything from ancient ships to graffitti art, and the Museum from Music Box to Fairground Organ (a very rough translation of the Dutch name), where you take a one hour tour and see the various mechanical music-makers demonstrated. Utrecht also has unusual bi-level canals.
Leiden (which you will pass through on the way to Keukenhof if you're going to the flower exhibits there) has a great college town feel (Delft and Utrecht also have universities, but do not look or feel like college towns). It has an interesting science museum (did you know that EKG's and renal dialysis were from the Netherlands?). I also enjoyed just walking around, and wish I had allotted more time there. To avoid offending others, I'll just say that I wasn't as taken with Keukenhof as most are.
Rotterdam feels very different from other Dutch cities. It was destroyed in WWII, and has lots of fascinating modern architecture (get the architecture walking tour brochure from the tourist office, which also doubles as a free map; they charge for a map otherwise). Don't miss the Cube House Museum - fascinating.
If you're seeing more than a few museums in the Netherlands, the Museumkaart is a great deal, as it's good all over the country.
Because it's tulip season, spring is peak time in Amsterdam - get your hotel reservations EARLY.