Things To Do
Keukenhof HOLLAND
This is one of the largest flower gardens in the world. It's only open in the Spring, and it's been closed for the previous two years.
Amsterdam
If you like old cities, you could spend your entire trip here. Amsterdam has history, art, food, museums, the Royal Palace, Heineken, and more. There are countless boat/walking/cycling/bus trips that originate there. In addition, the Tulip Festival will be going on!
Alkmaar
Alkmaar itself is charming and worth checking out. The big church (Sint Laurenskerk) is free to the public (largest windows in the Netherlands, according to the tour). Biking around is a Dutch activity in and of itself as well. Have some coffee/wine in the central square. Every Saturday, there's a street market.
Flower Parade
There's a flower parade on Friday evening and Saturday (23-23 Apr). It ends in Haarlem, which is a large, interesting city to visit in its own right.
Nearby Countryside
The countryside is lovely. There are windmills, castles, beaches, and fields in the middle of nowhere full of tulips. Some nearby towns are quaint as well (Edam/Bergen/Egmond aan Zee).
King's Day
27 April is King's Day. It's a national holiday, and there are festivals in all the larger cities!