In Belgium, I particularly enjoy Bruges (Brugge) and Antwerp (much prefer over Brussels). Bruges is like a Renaissance Disneyland, and Antwerp is just wonderful for sitting out on the terraces on the squares and drinking Trappist beer....beautiful city. I've never done it, but you can also visit a few monasteries where the Trappist beer is made (such as Westmalle, near Antwerp, or Chimay, in the south of Belglium). Trappist beer is the best! (I am afraid that the wines of Belgium and Holland are getting better and better but nothing to write home about, and also relatively expensive for what you get.)
In Holland, there's a few things, depending on what you like to do (and almost all websites are in English as well as Dutch):
Keukenhof in Lisse (keukenhof.nl): This is a completely fabulous garden, only open in sping, which should be full of tulips as well as other flowers in April. The plantings are amazing, like colorful flower paintings. A must for gardening fans.
Koningsdag, or KIng's Day, on Saturday, April 26. If you are in Holland on this day, you won't be able to avoid Koningsdag. The Dutch go nuts and the city centers are given over to bands playing, the streets lined with folks selling their items (like a street garage sale - this happens because it's the only day you can sell stuff legally and not have to pay value added tax on what you've sold), and massive beer drinking. This year will be the first King's Day so it should be especially interesting. Willen-Alexander took over the throne from his mom Beatrix last year on Queen's Day, so this will be his first King's DAy. He and his Argentinian wife Maxima will visit two towns along with the other royals and it's kind of a country-wide carnaval atmosphere. It's pretty fun/interesting to see the Dutch at their most overt and enthusiastic.
Kroller Muller Museum in Arnhem/Otterlo is super indeed. There are two world-renowned Van Gogh collections - one is in Amsterdam's Van Gogh Museum and the other is here. You can get up close to the paintings and really take a look at them at the Kroller Muller Museum - much less busy than the VG Museum. And it is situated in a sculpture park/garden that in itself is very worthwhile. Also in the park, you can tour the house built by the Kroller Muller's.
Also in Arnhem is the Openlucht or Open Air Museum (openairmuseum.nl). It's a lovely park with lots of Dutch history, like old Dutch villages, old farmhouses, and lots of activities. It is always open for just walking in the park, I think, and opens on April 1 for the full experience of Holland in the old days. Arnhem also has a lot of war-related things, due to Operation Market Garden, if that's interesting to you. Like the Airborne Museum in Oosterbeek (en.airbornemuseum.nl). And of course the 'Bridge Too Far' in Arnhem (John Frost Bridge).
There are lots of picturesque villages and towns all over Holland. My favorite cities are Amsterdam and Maastricht.
Enjoy your trip!