PalenQ

when I said a van I meant any vehicle that legally could accommodate your group.

the word van being ambiguous - van, mini-bus, coach, 15 bicycles with a leader, etc.

and yes like spaarne confirms it is very very easy to take the train to Leiden - the shuttle bus to Keukenhof then bus whatever number to slice thru the flower fields to Haarlem to take the train back to Amsterdam.

You'd have time to look around dreamy Haarlem a bit as well - all for nearly nothing compared to cost of hiring "van" or whatever - folks would not have to go as a huge group either but in smaller groups or on their own. Once at the gardens there is no need for a guide - everything is in English and self-explanatory.