The promotional material states that the 'Keukenhof is the largest spring garden in the whole world' with more than 7 million flowering bulbs. It is truly spectacular, you can spend a whole day just wandering around the gardens. There is also a kids' play area and various pavillions which put on special display. Unfortunately it is quite expensive, especially for a garden, which is why I haven't given it a higher recommendation. In addition, given the entry cost having to pay for the toilets in the park just seemed a bit much.
Practicalities: for 2003 the entry fee is 11,50 euro or 15,50 with a round trip bus ticket from Leiden. If taking the bus from the station, you need bus #54. I understand that there is also a bus from Haarlem. In
2003 the park is open 21 March to 18 May, so you may need to visit in 2004.
Their website is www.keukenhof.com